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

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2022
commit: https://github.com/kijiproject/kiji-phonebook/commit/db06cc71be7c019e8bf88dd0c0642adb715712e4