cassandra-maven-plugin: CMP-12. Cassandra Maven plugin should depend on Guava 15

issue: https://jira.kiji.org/browse/CMP-12
review: https://review.kiji.org/r/2103
commit: https://github.com/kijiproject/cassandra-maven-plugin/commit/9f1ff520444ee72550580dd884979505840fe648