kiji-model-repository: KIJIREPO-64. Fix compilation issues with Java 1.6

issue: https://jira.kiji.org/browse/KIJIREPO-64
review: https://review.kiji.org/r/1993
commit: https://github.com/kijiproject/kiji-model-repository/commit/2453f8eff30bf02d089c86138128c27d3ab39f62