Correcting functions signatures for MediaManager

Hi !

I just updated my code using composer, and the last modifications for sonata media manager arrived.
But now, I got this error on my website : 

    FatalErrorException: Compile Error: Declaration of Sonata\MediaBundle\Model\MediaManager::findBy() must be compatible with Sonata\CoreBundle\Entity\ManagerInterface::findBy(array $criteria, array $orderBy = NULL, $limit = NULL, $offset = NULL) in /var/www/manuel/htdocs/avocadoshop/vendor/sonata-project/media-bundle/Sonata/MediaBundle/Model/MediaManager.php line 17

It seems related to the PR #454 

My PR try to correct this issue.
do not hesitate to correct my correction :)