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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2025
commit: https://github.com/kijiproject/kiji-schema/commit/b34a93ada94906d9b0e734c24e511e5b3a85693e