[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.

The hector library was removed, because it didn't seem to work correctly
with C* 2.1.