kiji-mapreduce: KIJIMR-259. invoking kiji pivot causes NPE

 - KijiPivot.validateFlags() must invoke super.validateFlags().

issue: https://jira.kiji.org/browse/KIJIMR-259
review: https://review.kiji.org/r/1529
commit: https://github.com/kijiproject/kiji-mapreduce/commit/7aa4282bef5505e7e340b8b404f3141ca8df171c