updated VERSION for 2.3.23
bumped Symfony version to 2.3.24
CS fixes
Removed unused imports
Merge branch '2.3' into 2.5

* 2.3:
  [2.3] More cs fixes
  Removed unused imports
  CS fixes
  bumped Symfony version to 2.3.24
  updated VERSION for 2.3.23
  update CONTRIBUTORS for 2.3.23
  updated CHANGELOG for 2.3.23

Conflicts:
	src/Symfony/Bundle/FrameworkBundle/EventListener/SessionListener.php
	src/Symfony/Bundle/SecurityBundle/DependencyInjection/MainConfiguration.php
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/HttpKernel/Tests/Bundle/BundleTest.php
	src/Symfony/Component/Routing/Matcher/Dumper/PhpMatcherDumper.php