- Remove diamond inheritance tests since these test a bug in Django,
not in django_polymorphic.
- Fix spurious print message in test_primary_key_custom_field_problem(),
where uuid instances are for some reason being compared to uuid.UUID
classes. Replaced with a test that checks that the primary key values
are valid UUIDs.
- Remove Django > 1.6 conditionals wrapping the multiple inheritance
tests and model definitions, in favor of using the definitions for
Django 1.7. Django 1.7 requires a custom primary key when using concrete
model inheritance because failing to do so is potentially ambiguous. Adding
the custom primary key does not break tests in older Django versions.
