Update Symfony reference to 2.7 version

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.7 only
| Fixed tickets | 

It's a bit unpractical having to update the version by hand each time a mayor version is released... and hard to remember.

Can this be resolved with a parameter (or something similar) common to the whole documentation??