[Contributing] extended Symfony 2.4 maintenance

Maintenance for Symfony 2.4 has been extended from July 2014 to
September 2014.
minor #4101 [Contributing] extended Symfony 2.4 maintenance (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Contributing] extended Symfony 2.4 maintenance

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Maintenance for Symfony 2.4 has been extended from July 2014 to September 2014 (see [the blog post](http://symfony.com/blog/extended-maintenance-for-symfony-2-4)).

Commits
-------

92c67b5 [Contributing] extended Symfony 2.4 maintenance