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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2010
commit: https://github.com/kijiproject/kiji-mapreduce/commit/9f1e526ea29f2d84c4314b88a7e44dc203a28139