[cassandra] Bump to current stable version.

The 2.0.0-rc2 driver has a bug
(https://datastax-oss.atlassian.net/browse/JAVA-254) which gets
triggered when querying through presto.

The exception occurs because the driver attempts to query a table that
no longer exists, and fails.

Cassandra 2.1.0 is the current stable release, so it's probably time to
catch up.