diff --git a/composer.json b/composer.json
index 8fe2f28..c6ce459 100644
--- a/composer.json
+++ b/composer.json
@@ -19,7 +19,7 @@
     "twig/twig": "1.15.*",
     "doctrine/common": "dev-master#a45d110f71c323e29f41eb0696fa230e3fa1b1b5",
     "doctrine/annotations": "1.2.*",
-    "guzzlehttp/guzzle": "4.1.*",
+    "guzzlehttp/guzzle": "~5.0",
     "kriswallsmith/assetic": "1.1.*@alpha",
     "symfony-cmf/routing": "1.2.*",
     "easyrdf/easyrdf": "0.8.*",
diff --git a/composer.lock b/composer.lock
index 371f59e..98e8627 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at http://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "hash": "84084dce2bc995cf540fa58e5f3c796d",
+    "hash": "27aa103761ed4c03caa9b50bb19db230",
     "packages": [
         {
             "name": "doctrine/annotations",
@@ -510,21 +510,20 @@
         },
         {
             "name": "guzzlehttp/guzzle",
-            "version": "4.1.7",
+            "version": "5.0.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/guzzle/guzzle.git",
-                "reference": "448f2c2076cf0fb756230611491c4f7ecb735a29"
+                "reference": "28b51e11237f25cdb0efaea8e45af26007831aa9"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/guzzle/guzzle/zipball/448f2c2076cf0fb756230611491c4f7ecb735a29",
-                "reference": "448f2c2076cf0fb756230611491c4f7ecb735a29",
+                "url": "https://api.github.com/repos/guzzle/guzzle/zipball/28b51e11237f25cdb0efaea8e45af26007831aa9",
+                "reference": "28b51e11237f25cdb0efaea8e45af26007831aa9",
                 "shasum": ""
             },
             "require": {
-                "ext-json": "*",
-                "guzzlehttp/streams": "~1.4",
+                "guzzlehttp/ringphp": "~1.0",
                 "php": ">=5.4.0"
             },
             "require-dev": {
@@ -532,22 +531,16 @@
                 "phpunit/phpunit": "~4.0",
                 "psr/log": "~1.0"
             },
-            "suggest": {
-                "ext-curl": "Guzzle will use specific adapters if cURL is present"
-            },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "4.1-dev"
+                    "dev-ring": "5.0-dev"
                 }
             },
             "autoload": {
                 "psr-4": {
                     "GuzzleHttp\\": "src/"
-                },
-                "files": [
-                    "src/functions.php"
-                ]
+                }
             },
             "notification-url": "https://packagist.org/downloads/",
             "license": [
@@ -571,20 +564,70 @@
                 "rest",
                 "web service"
             ],
-            "time": "2014-08-08 01:30:43"
+            "time": "2014-10-13 03:05:51"
+        },
+        {
+            "name": "guzzlehttp/ringphp",
+            "version": "1.0.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/guzzle/RingPHP.git",
+                "reference": "9e44b565d726d9614cd970319e6eea70ee15bff3"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/guzzle/RingPHP/zipball/9e44b565d726d9614cd970319e6eea70ee15bff3",
+                "reference": "9e44b565d726d9614cd970319e6eea70ee15bff3",
+                "shasum": ""
+            },
+            "require": {
+                "guzzlehttp/streams": "~3.0",
+                "php": ">=5.4.0",
+                "react/promise": "~2.0"
+            },
+            "require-dev": {
+                "ext-curl": "*",
+                "phpunit/phpunit": "~4.0"
+            },
+            "suggest": {
+                "ext-curl": "Guzzle will use specific adapters if cURL is present"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "1.0-dev"
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "GuzzleHttp\\Ring\\": "src/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Michael Dowling",
+                    "email": "mtdowling@gmail.com",
+                    "homepage": "https://github.com/mtdowling"
+                }
+            ],
+            "time": "2014-10-13 00:59:38"
         },
         {
             "name": "guzzlehttp/streams",
-            "version": "1.5.1",
+            "version": "3.0.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/guzzle/streams.git",
-                "reference": "fb0d1ee29987c2bdc59867bffaade6fc88c2675f"
+                "reference": "47aaa48e27dae43d39fc1cea0ccf0d84ac1a2ba5"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/guzzle/streams/zipball/fb0d1ee29987c2bdc59867bffaade6fc88c2675f",
-                "reference": "fb0d1ee29987c2bdc59867bffaade6fc88c2675f",
+                "url": "https://api.github.com/repos/guzzle/streams/zipball/47aaa48e27dae43d39fc1cea0ccf0d84ac1a2ba5",
+                "reference": "47aaa48e27dae43d39fc1cea0ccf0d84ac1a2ba5",
                 "shasum": ""
             },
             "require": {
@@ -596,16 +639,13 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.5-dev"
+                    "dev-master": "3.0-dev"
                 }
             },
             "autoload": {
                 "psr-4": {
                     "GuzzleHttp\\Stream\\": "src/"
-                },
-                "files": [
-                    "src/functions.php"
-                ]
+                }
             },
             "notification-url": "https://packagist.org/downloads/",
             "license": [
@@ -618,13 +658,13 @@
                     "homepage": "https://github.com/mtdowling"
                 }
             ],
-            "description": "Provides a simple abstraction over streams of data (Guzzle 4+)",
+            "description": "Provides a simple abstraction over streams of data",
             "homepage": "http://guzzlephp.org/",
             "keywords": [
                 "Guzzle",
                 "stream"
             ],
-            "time": "2014-08-10 23:57:01"
+            "time": "2014-10-12 19:18:40"
         },
         {
             "name": "kriswallsmith/assetic",
@@ -1153,6 +1193,52 @@
             "time": "2012-12-21 11:40:51"
         },
         {
+            "name": "react/promise",
+            "version": "v2.0.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/reactphp/promise.git",
+                "reference": "58129a9cb9da88f2055309a805e2696b06928bb0"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/reactphp/promise/zipball/58129a9cb9da88f2055309a805e2696b06928bb0",
+                "reference": "58129a9cb9da88f2055309a805e2696b06928bb0",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=5.4.0"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "2.0-dev"
+                }
+            },
+            "autoload": {
+                "psr-0": {
+                    "React\\Promise": "src/"
+                },
+                "files": [
+                    "src/React/Promise/functions.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Jan Sorgalla",
+                    "email": "jsorgalla@googlemail.com",
+                    "homepage": "http://sorgalla.com",
+                    "role": "maintainer"
+                }
+            ],
+            "description": "A lightweight implementation of CommonJS Promises/A for PHP",
+            "time": "2013-12-10 15:40:36"
+        },
+        {
             "name": "sdboyer/gliph",
             "version": "0.1.4",
             "source": {
@@ -2483,12 +2569,8 @@
             "time": "2013-06-12 19:46:58"
         }
     ],
-    "packages-dev": [
-
-    ],
-    "aliases": [
-
-    ],
+    "packages-dev": [],
+    "aliases": [],
     "minimum-stability": "stable",
     "stability-flags": {
         "symfony/yaml": 20,
@@ -2498,9 +2580,7 @@
     },
     "prefer-stable": false,
     "platform": {
-        "php": ">=5.4.2"
+        "php": ">=5.4.5"
     },
-    "platform-dev": [
-
-    ]
+    "platform-dev": []
 }
diff --git a/core/modules/update/src/UpdateFetcher.php b/core/modules/update/src/UpdateFetcher.php
index 14bdb29..5f22265 100644
--- a/core/modules/update/src/UpdateFetcher.php
+++ b/core/modules/update/src/UpdateFetcher.php
@@ -8,6 +8,7 @@
 namespace Drupal\update;
 
 use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\DependencyInjection\DependencySerializationTrait;
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\RequestException;
 
@@ -16,6 +17,8 @@
  */
 class UpdateFetcher implements UpdateFetcherInterface {
 
+  use DependencySerializationTrait;
+
   /**
    * URL to check for updates, if a given project doesn't define its own.
    */
diff --git a/core/modules/update/src/UpdateProcessor.php b/core/modules/update/src/UpdateProcessor.php
index d37f643..f129845 100644
--- a/core/modules/update/src/UpdateProcessor.php
+++ b/core/modules/update/src/UpdateProcessor.php
@@ -161,7 +161,7 @@ public function processFetchTask($project) {
     $project_name = $project['name'];
 
     if (empty($this->failed[$fetch_url_base]) || $this->failed[$fetch_url_base] < $max_fetch_attempts) {
-      $data = $this->updateFetcher->fetchProjectData($project, $site_key);
+      $data = (string) $this->updateFetcher->fetchProjectData($project, $site_key);
     }
     if (!empty($data)) {
       $available = $this->parseXml($data);