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.
okay, if we can be confident about that. if i got @Ocramius correctly, he wants to wait for php 5.6 to be stable - not sure how long this is going to take... but we can still fix at that point, or ask for a beta release or something.