make PHPCR ODM 5.6 compatible

fixing the bug with creating new class instances

fixes https://github.com/doctrine/phpcr-odm/issues/534

however we need to wait for Doctrine to release a stable version but imho we should already merge this.
@lsmith77 it's still a library, so I don't want people to rely on a concrete implementation unless they are really sure about what they are doing.