Test components using their lowest possible deps
minor #12542 Test components using their lowest possible deps (nicolas-grekas)

This PR was squashed before being merged into the 2.3 branch (closes #12542).

Discussion
----------

Test components using their lowest possible deps

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Once https://github.com/composer/composer/pull/3450 is merged, we'll see if our deps are correct, and ensure that is kept over time by testing each component with its lowest possible deps.

Commits
-------

25fef27 Test components using their lowest possible deps
Merge remote-tracking branch 'symfony/2.3' into lowest-25

* symfony/2.3:
  Test components using their lowest possible deps

Conflicts:
	src/Symfony/Bridge/Doctrine/composer.json
	src/Symfony/Bridge/Twig/composer.json
	src/Symfony/Bundle/FrameworkBundle/composer.json
	src/Symfony/Bundle/SecurityBundle/composer.json
	src/Symfony/Bundle/TwigBundle/composer.json
	src/Symfony/Component/DependencyInjection/composer.json
	src/Symfony/Component/EventDispatcher/composer.json
	src/Symfony/Component/Form/composer.json
	src/Symfony/Component/HttpKernel/composer.json
	src/Symfony/Component/Routing/composer.json
	src/Symfony/Component/Security/composer.json
	src/Symfony/Component/Validator/composer.json
Test lowest versions of dependencies
minor #12998 [2.5] Test components using their lowest possible deps (nicolas-grekas, fabpot)

This PR was merged into the 2.5 branch.

Discussion
----------

[2.5] Test components using their lowest possible deps

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | not yet
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Follow up of #12542

Commits
-------

b1b5cca Test lowest versions of dependencies
88d524e Merge remote-tracking branch 'symfony/2.3' into lowest-25
206ebc7 minor #12542 Test components using their lowest possible deps (nicolas-grekas)
25fef27 Test components using their lowest possible deps