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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2023
commit: https://github.com/kijiproject/kiji-platforms/commit/ac05fc345701bdd3547ff6e8f10432dd41f38576