kiji-poms: POM-37. Update dependency plugin to 2.8 for newer maven compatibility

issue: https://jira.kiji.org/browse/POM-37
review: https://review.kiji.org/r/1936
commit: https://github.com/kijiproject/kiji-poms/commit/4cdeba5529a54e40be6146b39e2a6f621aa1943d