Update Symfony components to 3.2.8

Don't want to disturb the RTBC patch on #2712647: Update Symfony components to ~3.2, but 3.2.8 came out today. Here is that patch with ~3.2 and then `composer update 'symfony/*'`, to get 3.2.8 in the lockfile.

Added this issue to meta issue #2864037: [META] Update core PHP dependencies. #2712647: Update Symfony components to ~3.2 is in so the patch needs a reroll.

With Symfony 3.3.0 due for release later this month I have opened #2874909: Update Symfony components to 3.3.*.

OK, fresh start to get back to ~3.2 in core/composer.json and `composer update 'symfony/*'` to get 3.2.8 in the lockfile.

Excerpt of my bash history where I verified this patch:

$ sed -i 's/3.2.6/\~3.2/' core/composer.json
$ composer update symfony/*
...
$ curl https://www.drupal.org/files/issues/2871253-06.patch | git apply -R --check
$ # No output, yay! ;-)

Another library update #2874817: Update zendframework/zend-diactoros from 1.3.10 to 1.4.0.

Reroll b/c tiny conflict from #2488860: Bring phpunit bridge into drupal and use it for unit tests and simpletest to handle Deprecation.

Heads up: We're working on how to achieve parity between Components' requirement constraints and changes to core here: #2867960: Merge Component composer.json files to account for them during build

Steps from #7 still work with #10.

Related change record: https://www.drupal.org/node/2891254

Related to: #2887000: composer.json does not constrain Symfony components to minor and patch versions that are compatible with Drupal