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/2006
commit: https://github.com/kijiproject/kiji-delegation/commit/d700de10e5ba69eb7dfd12f536dfd4a8b9544415