bump Scala to 2.12.6 and 2.13.0-M4

Author: SethTisue

build.sbt

@@ -2,8 +2,8 @@ import ScalaModulePlugin._
 
 scalaVersionsByJvm in ThisBuild := {
   val v211 = "2.11.12"
-  val v212 = "2.12.4"
-  val v213 = "2.13.0-M3"
+  val v212 = "2.12.6"
+  val v213 = "2.13.0-M4"
   Map(
     6 -> List(v211 -> true),
     7 -> List(v211 -> false),