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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2009
commit: https://github.com/kijiproject/kiji-hive-adapter/commit/b5ad2e3cf9324a76f74a20b3166c868f33d16467