kiji-bento: BENTO-81. Specify version of Maven plugins through root pom managed plugins.

issue: https://jira.kiji.org/browse/BENTO-81
review: https://review.kiji.org/r/1920
commit: https://github.com/kijiproject/kiji-bento/commit/b3e44a61323b6a6785f1186ab5b3f5a8d1eb8785