cassandra-maven-plugin: CMP-7. Add group ID for maven-invoker-plugin

issue: https://jira.kiji.org/browse/CMP-7
review: https://review.kiji.org/r/2059
commit: https://github.com/kijiproject/cassandra-maven-plugin/commit/3377e0d676d45ab00ef8a0df3854377fd522432f