PartialObjectExpression and the PARTIAL DQL grammar are absent from doctrine/orm 3.0.0 through 3.2.3 and return in 3.3.0. Raise the floor in both the component and the root composer.json, drop the now-dead class_exists() guard, and require doctrine/doctrine-bundle ^2.11.1 (2.11.0 passes $reportFieldsWhereDeclared to AttributeDriver, which ORM 3 rejects).

Also converts EagerLoadingExtensionTest from Prophecy to PHPUnit mocks.