Upgrade to SBT 0.13

The upgrade will allow us to use some newer plugins, especially the [sbt-scoverage](https://github.com/scoverage/sbt-scoverage) plugin for code coverage.

I debated whether to cross-publish this plugin for both 0.12 and 0.13, but this is a pain.  There is a dependency to help do this (sbt-cross-building), but it requires a different command to publish, which is different than regular libraries, and seems too much of a pain to modify a release process we just established.