Fix tracing.

Celery replaces the default logger returned with ProcessAwareLogger, so
we no longer recognised logger classes based on the primitive logger
class.

inspect.isclass(karaage.datastores.gold.logger) returns false despite it
being an instance of logging.Logger, this check is probably faulty.

Change-Id: I5b226efcb7883e84730905763874a43c39f2356f