Guzzle mock plugin is not included

Our composer.json file currently includes only the base Guzzle package. That's OK, generally speaking, but one package it leaves out is the mock plugin:

https://packagist.org/packages/guzzle/plugin-mock

Without that, we cannot easily test guzzle-using code. That plugin makes it easy to set mock responses, which means we can run tests without a network connection (good). We should include that by default to ease testability. (I just ran into this myself trying to test a demo module I'm writing.)

http://docs.guzzlephp.org/en/latest/testing/unit-testing.html