Use EntityManagerInterface instead of EntityManager (to support decorators, etc)

If you use [EntityManagerDecorator](http://www.doctrine-project.org/api/orm/2.4/class-Doctrine.ORM.Decorator.EntityManagerDecorator.html) or any other entity manager that isn't a Doctrine\ORM\EntityManager class or subclass, then the fixtures break.  Instead, it should use EntityManagerInterface.