Update ciql gem dependency

* Ciql changed the underlying Cassandra driver, so some changes
  to the adapter were needed to compensate.
Update adapter to connect to configured host and keyspace
Change #execute to return nil to discourage return value usage

* In dm-do-adapter the #execute method returns the number of
  affected rows, but that isn't possible with Cassandra so return
  nil to discourage relying on the return value for anything
  meaningful.