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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2000
commit: https://github.com/kijiproject/kiji-poms/commit/7d6d54bbc0b441d9ad2bdde717c04333196e3077