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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2026
commit: https://github.com/kijiproject/kiji-schema-shell/commit/d8ae973c3e492d86dcd4ca07a39bc00156d468f2