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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2019
commit: https://github.com/kijiproject/kiji-model-repository/commit/811f488939cb5273e1994edd3e2de17271f51cc4