Update Cassandra configurations to latest.

When upgrading to Cassandra 2.0, we forgot to upgrade the configuration files included.

This fix is relevant to travis-ci/travis-ci#1650.

This uses the vanilla files from 2.0.2. I verified that Cassandra works with them, unmodified, on an IPv6 box.

There's one caveat, Cassandra 2.0.x doesn't seem to work on JDK 6 anymore.

There are a few open tasks:

- [ ] Verify with a library and possibly an app that it actually works
- [ ] Patch the hosts file in travis-build to add in a line with 127.0.0.1 and an actual hostname