kiji-bento: BENTO-77.  Use python3 for redundant-JAR script.

review: https://review.kiji.org/r/1897
issue: https://jira.kiji.org/BENTO-77
commit: https://github.com/kijiproject/kiji-bento/commit/871ce2b9b686df2a10d4a457e704e0a07568295b