minor cleanup of fork
Allowed a :scala-version to be specified as per tomwans' fork here:

https://github.com/tomwans/lein-scalac/blob/8efbe9165e9d7424b4b3cbdecc6fe8a84f56c6ef/src/leiningen/scalac.clj#L11

This is different in that the dependencies are provided through
leiningen middleware, which provides for scala-lang.org
dependencies to not be respecified in the project.clj.