Make rad entity repository concrete

As from KnpLabs/rad-edition@759bcc44ca1ec5a13c7dbf8727d8edd587c35e46
Rad EntityRepository is the default one, it needs to be concrete or the
EntityManager won't be able to instanciate it when the user do not use
a concrete custom repository.

See
https://github.com/doctrine/doctrine2/blob/2.3.2/lib/Doctrine/ORM/EntityManager.php#L689