cross-publish by minor Scala version

(we don't publish this plugin at all, but for demonstration
purposes...)

fixes #1

build.sbt

@@ -3,6 +3,7 @@ version := "0.0.1"
 name := "cloc-plugin"
 scalaVersion := crossScalaVersions.value.head
 crossScalaVersions := Seq("2.12.6", "2.13.0-M4")
+crossVersion := CrossVersion.patch
 libraryDependencies +=
   "org.scala-lang" % "scala-compiler" % scalaVersion.value % "provided"
 scalacOptions ++= Seq(