fix ocramius/proxy-manager dependency version

Symfony 2.5 and higher requires ocramius/proxy-manager 0.4.0 or
higher.
Fix failing tests after merge
minor #12967 Fix failing tests after merge (xabbuh, nicolas-grekas)

This PR was merged into the 2.5 branch.

Discussion
----------

Fix failing tests after merge

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

https://github.com/symfony/symfony/pull/12967/files?w=1

Commits
-------

9b96373 Fix failing tests after merge
6638535 fix ocramius/proxy-manager dependency version