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