compare version using PHP_VERSION_ID

This continues the work started in #12497 on the `2.3` branch.
Rename Symfony2 to Symfony
Merge branch '2.3' into 2.5

* 2.3:
  [Debug] fix error message on double exception
  Fix initialized() with aliased services
  Rename Symfony2 to Symfony
  backport #12489

Conflicts:
	CONTRIBUTING.md
	src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php
	src/Symfony/Bundle/TwigBundle/Loader/FilesystemLoader.php
Merge branch '2.5' into 2.6

* 2.5:
  [Debug] fix error message on double exception
  Fix initialized() with aliased services
  Rename Symfony2 to Symfony
  compare version using PHP_VERSION_ID
  backport #12489
  remove an unneeded check
Merge branch '2.6' into 2.7

* 2.6: (36 commits)
  [Debug] fix error message on double exception
  [Validator] make DateTime objects represented as strings in the violation message.
  [RFC] [DebugBundle] [HttpKernel] Avoid using container as dependency for DumpListener
  Upgrade information for the Translation component regarding the new LoggingTranslator class.
  [WebProfilerBundle] Remove usage of app.request in search bar template
  Fix initialized() with aliased services
  fix data type in docblock
  Rename Symfony2 to Symfony
  bumped Symfony version to 2.6.0
  updated VERSION for 2.6.0-BETA2
  updated CHANGELOG for 2.6.0-BETA2
  [Debug] fix ENT_SUBSTITUTE usage
  compare version using PHP_VERSION_ID
  backport #12489
  remove an unneeded check
  Remove block submit_widget
  reformat code as suggested by @fabpot
  Fix typo
  Make `\Request::get` more performant.
  properly set request attributes in controller test
  ...