use sbt's recommended publication approach

@aemoncannon please follow the signup process here for bintray: create the `sbt-plugins` repo, publish this (add a git tag `v0.1.4` first), and then add that artefact to the sbt plugin distros.

http://www.scala-sbt.org/0.13.5/docs/Community/Bintray-For-Plugins.html

then this PR will allow really simple dists of the sbt plugin without the sonatype staging faff.