kiji-express: EXP-434. Update REVIEWERS file.

issue: https://jira.kiji.org/browse/EXP-434
review: https://review.kiji.org/r/1814
commit: https://github.com/kijiproject/kiji-express/commit/881072d1321a2d3294e75fe0a79efe3df9e7b0e0