Try to update to 2.1.5 (ONLY MERGE WHEN GREEN)
Fix a bug Chris pointed out: don't keep CTuples
Improve Tuple copying
Work around for new AggregateBy behavior
Also remove the comparator in from Fields in MRM
Merge pull request #340 from twitter/feature/cascading-2.1

Try to update to 2.1.5 (ONLY MERGE WHEN GREEN)