kiji-checkin: POM-40. Require JDK 7 to compile Kiji projects

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2005
commit: https://github.com/kijiproject/kiji-checkin/commit/2ba1a1e87f9d36bb77fae6b0423cc1011ea60e17