Enable coverage for cassandra.

.coveragerc

@@ -16,7 +16,6 @@ omit =
     *celery/contrib/sphinx.py
     *celery/backends/couchdb.py
     *celery/backends/couchbase.py
-    *celery/backends/cassandra.py
     *celery/backends/riak.py
     *celery/concurrency/asynpool.py
     *celery/utils/debug.py