Upgrade to sbt 0.13.1.

Upgrade zinc project itself to sbt 0.13.1. The only breaking change was
removal of sbt.Util.counted method in 0.13. I just copied those few lines
of code from the old version of sbt directly to zinc.
Bump sbt version zinc depends on to 0.13.2-SNAPSHOT

This is the version that zinc uses to build its own components. It's
not the version used for the project itself.