Enable compatibility and testing with Symfony 2.4
merged branch romainneutron/symfony-2.4 (PR #764)

This PR was squashed before being merged into the master branch (closes #764).

Discussion
----------

Enable compatibility and testing with Symfony 2.4

Hello,

Symfony 2.4 is scheduled for november then, according to Symfony release process, the stabilization phase should start in september.

I think it's a good idea to add Symfony 2.4 compatibility as it seems there's no breakage and some people would like to work on the 2.4 branch while working on Silex.

Commits
-------

0819b49 Enable compatibility and testing with Symfony 2.4