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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2017
over-the-shoulder-reviewers: ['lsheng']
commit: https://github.com/kijiproject/kiji-common-flags/commit/5fc4fd978e1b4a6495d19d18ef0e2627a4c838f5