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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2028
commit: https://github.com/kijiproject/kiji-scoring-server/commit/09e91dae6dd76b17af075ee781f9fdac66541e28