Reject invalid subclassing of RLMObjects at schema generation time

Rather than producing weird errors and bizarre behavior when they're actually
used.

Remove the unit test which was testing them since there's no way to have an
invalid subclass for just a single test.
fixes for compiler warnings in testS
Merge pull request #775 from realm/al-quick-fix

fixes for compiler warnings in tests
Merge pull request #769 from realm/tg-rlmboject-indirect-subclass

Reject invalid subclassing of RLMObjects at schema generation time
Bump the core version