Fix change w.r.t overriding API's change of semantics in Scala's 0eff6cd

overridenSymbol became assymetric and does not reports abstract that look like they would override a concrete.
it was used here in a symmetric manner (i.e. reporting overridees as well as overrides)
Fixing test w.r.t. 2.11.x branch
Merge pull request #10 from huitseeker/issue/2.11.x-compatibility

Fix tests with the 2.11 branch of the Scala compiler