Remove the ORM from composer's "require" in favor of ODM uses

[dorongutman]

doctrine/data-fixtures supports the mongodb ODM as well as the ORM.
Currently the composer.json file requires the ORM which causes the ORM to download even if I don't use it, but uses the ODM.

[Ocramius]

@dorongutman can you open a PR for this?

[fabiocarneiro]

It wont work anyway. EntityManager and ORMPurger are hardcoded in this module.