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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2020
commit: https://github.com/kijiproject/kiji-modeling/commit/ea99ef60200d0a98f0b70ff34297c1e9b4c3a2c8