[storm-cassandra] upgrade cassandra driver to 3.1.2

Currently, storm-cassandra refers cassandra driver 2.1.7.1 which is not compatible with Cassandra 3.x.

Fortunately cassandra driver 3.1.2 is compatible with various Cassandra versions (Apache Cassandra 1.2, 2.0, 2.1, 2.2 and 3.0), so upgrade driver should be safe for users who uses prior version of Cassandra 3.0.
http://docs.datastax.com/en/developer/java-driver/3.1/#compatibility