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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2016
over-the-shoulder-reviewers: ['lsheng']
commit: https://github.com/kijiproject/kiji-delegation/commit/4dafeee7e1f0437b3b04c9d6f775eab3276b07fc