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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2004
commit: https://github.com/kijiproject/kiji-bento/commit/9281ff260fa118dcd7d0fcc356b8110e826840e0