Update Symfony reference to 2.6 version

Update Symfony reference to from 2.5 to 2.6 version
minor #4568 Update Symfony reference to 2.6 version (dangarzon)

This PR was merged into the 2.6 branch.

Discussion
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Update Symfony reference to 2.6 version

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6 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??

Commits
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27cb445 Update Symfony reference to 2.6 version