Support for Doctrine ORM 2.5

[benr77]

The composer.json imposes a limit of "doctrine/orm": "~2.2,>=2.2.3" - is the upper bound strictly necessary?

Doctrine 2.5 offers a lot of improvements, and whilst it's not going to be the default in Symfony for ages (even 3.0), upgrading to it was painless with zero BC breaks in my relatively large Symfony project.

Any thoughts?

[mbiagetti]

👍