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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2008
commit: https://github.com/kijiproject/kiji-express-music/commit/2357f8e7cf9066d4a52ad6caf82634154b04367c