kiji-express: EXP-432. Protocol Buffer dependency should be in provided scope.

issue: https://jira.kiji.org/browse/EXP-432
review: https://review.kiji.org/r/1778
commit: https://github.com/kijiproject/kiji-express/commit/76dfe968ae57d7b826bcf44399a9923777a7f1b2