More changes towards sbt 0.13 syntax for build definition

The build definition has finally migrated to sbt 0.13 syntax (almost fully with one place where it was not obvious how to work it out) that effectively broke the **sbt-cross-building** plugin in the project.

I think it's time to branch off the repo for sbt 0.12 and sbt 0.13 development streams and there might be more breaking changes. I also think there's no point to maintain sbt 0.12 until there's one request asking for this.

References:
https://github.com/jrudolph/sbt-cross-building/issues/28
https://github.com/jrudolph/sbt-cross-building/issues/29