updated CHANGELOG for 2.3.12
update CONTRIBUTORS for 2.3.12
updated VERSION for 2.3.12
bumped Symfony version to 2.3.13
Merge branch '2.3' into 2.4

* 2.3:
  bumped Symfony version to 2.3.13
  updated VERSION for 2.3.12
  update CONTRIBUTORS for 2.3.12
  updated CHANGELOG for 2.3.12

Conflicts:
	src/Symfony/Component/HttpKernel/Kernel.php