Improve snapshot reducer calculation.

Currently too many reducers are being used, because we have a large data set being
compressed into a fairly small one, and we end up with 3000+ reducers all running
for a really short amount of time. This leads to a fragmented output and causes
issues for pivot as well.
Merge pull request #167 from ambiata/topic/faster

Improve snapshot reducer calculation.
Change FactsetId to use OldIdentifier instead of string
Add sonatype repo to plugins to hopefully fix Travis build