First commits

This PR contains 3 unrelated, small changes:
1. Point the build at a filesystem repo for work at Etsy (see below) and adjust the inclusion of algebird and chill for our purposes
2. Remove an unused import from a unit test that caused it to fail to compile against the version of algebird we require
3. Make use of the Averager MonoidAggregator and aggregate rather than implement it with AveragedValue and mapPlusMap

1 is the most concerning of these since it hard-codes the absolute path to the Etsy filesystem repository on my laptop.  It would be reasonable to reject this change until I figure out how to wade through sbt's setup to make this a configurable property of the build.