kiji-mapreduce-lib: KIJIMRLIB-28. Update REVIEWERS file.

issue: https://jira.kiji.org/browse/KIJIMRLIB-28
review: https://review.kiji.org/r/1818
commit: https://github.com/kijiproject/kiji-mapreduce-lib/commit/1b9b3df45ce4f37093b80e585a2896a61777119d