fix two dev dependencies that could lead to errors when running the tests

There were two issues with the old composer.json if you ran ``composer update``:

* The implementation of ``Doctrine\ORM\Proxy\Proxy`` changed between ``doctrine/orm`` 2.3 and 2.4 (doctrine/doctrine2@8272ffd23fd66bac13a0dc074c868a00b82c7707) which caused the ``SimpleObjectProxy`` fixture class not being valid any more.

* The constructor of the ``ConstraintViolation`` class got an additional argument ``$message`` with Symfony 2.2.