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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2018
commit: https://github.com/kijiproject/kiji-framework/commit/bdeaa4c9799b78852ef5bd50681d294ba2640b69