Run integration tests against cassandra 2.0

- Add profile to set cassandra maven plugin version to 2.0
- Run integration tests against cassandra 2.0 by default since that is
  what we will be running in the new prod environment.
Remove openjdk6 from travis test matrix

the cassandra 2.0 maven crashes when run against java 6.
Merge pull request #273 from dlobue/cassandra_2.0_integration_tests

Run integration tests against cassandra 2.0