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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2011
commit: https://github.com/kijiproject/kiji-mapreduce-lib/commit/c0c0c9228f81cd6ccf696263bd86090e01906027