Removed support for older Doctrine versions

The minimal versions are now Doctrine Common 2.3, Doctrine ORM 2.4 and Doctrine MongoDB ODM 1.0-alpha10.

I just figured out that CouchDB does not yet rely on the Doctrine Common events yet, so I will need to change this partially (I was in the train when doing it, without access to github to check it).
I'm wondering whether I revert the removal of the CouchDB dedicated listener, or whether I update the ODM and try to convince @beberlei to release a new version instead. I don't know how many people use the CouchDB integration in the bundle.