Compatible with Doctrine ORM >= 2.7

[peter-gribanov]

The doctrine/orm version 2.7 and later use doctrine/persistence package and uses Doctrine\Persistence\Proxy class instead of Doctrine\Common\Persistence\Proxy.

[coveralls]

Coverage decreased (-20.0%) to 80.0% when pulling 626b089 on peter-gribanov:doctrine_2.7 into 0342c65 on gpslab:master.

[coveralls]

Coverage remained the same at 100.0% when pulling ccef231 on peter-gribanov:doctrine_2.7 into 0342c65 on gpslab:master.

[coveralls]

Coverage remained the same at 100.0% when pulling ccef231 on peter-gribanov:doctrine_2.7 into 0342c65 on gpslab:master.