Update Cassandra versions in README.
Update Cassandra versions in README.

rightscale/cassandra,cassandra-rb/cassandra,stipple/cassandra_new,cassandra-rb/cassandra,stipple/cassandra_new,cassandra-rb/cassandra,rightscale/cassandra,cassandra-rb/cassandra,stipple/cassandra_new,stipple/cassandra_new,rightscale/cassandra