bento-cluster: POM-40. Require JDK 7 to compile Kiji projects

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2002
commit: https://github.com/kijiproject/bento-cluster/commit/a9bcee70e4416f05e257cc57ea8e0aba88f67565