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.
you mean it should not be possible to set a different instantiator at all? we could wrap it in a getter, to still allow extending ClassMetadata to use a different one maybe?