Support sbt 0.13

Here are updates for building and testing in sbt 0.13.  ls-sbt has to be changed back to 0.1.2 to build in sbt 0.12, and the scripted tests are 0.13-only, but everything else is sound for a cross-build, should you wish to preserve that option; I have left the deprecated `Project.Initialize` use in place for that reason.

If you'd prefer, I could see if [sbt-cross-building](https://github.com/jrudolph/sbt-cross-building) would help.  If you only plan to maintain builds for single versions of sbt in the future, though, there is not much point.

Thanks, and thanks for making this plugin!