bump Scala 2.12.4 -> 2.12.6

no special reason, just dogfooding

Author: SethTisue

build.sbt

@@ -4,7 +4,7 @@ scalaModuleSettings
 
 scalaVersionsByJvm in ThisBuild := {
   val v211 = "2.11.12"
-  val v212 = "2.12.4"
+  val v212 = "2.12.6"
   val v213 = "2.13.0-M3"
 
   Map(