[BLOCKED] upgrade to Symfony 2.5

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1755
| Refs tickets  | -
| License       | MIT
| Doc PR        | -


Please do not merge yet.

Blocked by:

* https://github.com/symfony/symfony/issues/11036 (PR: https://github.com/symfony/symfony/pull/11149)
* https://github.com/symfony/symfony/issues/11345