kiji-mapreduce: KIJIMR-247. Fixes test failure (org.kiji.mapreduce.TestGathererReducer) on OSX by increasing heap size for surefire.

issue: https://jira.kiji.org/browse/KIJIMR-247
review: https://review.kiji.org/r/1279
commit: https://github.com/kijiproject/kiji-mapreduce/commit/f8644ad6d35240afe1a56528abdbd597f181eb67