Scala major version should be set automatically when depending on Scala plugin

[mutcianm]

Due to Scala's binary incompatibility between major release versions, a situation may occur when the user sets project's scalaVersion to a different major version from the one used in Scala plugin for IJ.
While the compilation may succeed user plugin project will fail to run with cryptic classloading errors because due to IJ classloading mechanism scala-library from parent's classloader will be used and the version won't match.
See #11