let people pick their scalaVersion for build files

[cvogt]

currently build files use cbt's scalaVersion 2.11.8. Not all the dependencies of existing plugins exist for 2.12, so upgrading is not an option right now, but we could at least allow users to choose the scalaVersion. This would require CBT to allow being compiled and run in a different version, which should be similar to using entirely different versions in DirectoryDependency, but simply passing through a different version to be used.