Merge pull request #3 from adriaanm/master

Switch to scala-module-plugin, sbt 0.13.1
Fail fast in toolbox tests upon absent plugin JAR

To help us diagnose the likely cause of:

   https://github.com/retronym/community-builds/commit/964debeb52
Merge pull request #4 from retronym/topic/check-exists

Fail fast in toolbox tests upon absent plugin JAR
Plugin is fully cross-versioned because it depends on the compiler