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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2007
commit: https://github.com/kijiproject/kiji-express/commit/97c5bd40f343ae155dac322d942d47a85c90a474