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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2024
commit: https://github.com/kijiproject/kiji-rest/commit/6bd4c04c5f8b6ea0bc3255799dd33a70eb7959c7