Bump to sbt 0.13.

This is a work in progress. For the time being, to build this, you'll need to `publish-local` the latest snapshot of [sbt-yui-compressor](https://github.com/indrajitr/sbt-yui-compressor). Things appear to boot though yui is throwing out usage information which leads me to believe something is amiss. Haven't had time to check it out yet, but I'm opening a PR in the event someone else would like to.