Specify crossScalaVersions in the build.

Author: sjrd

build.sbt

@@ -1,10 +1,12 @@
 import sbt.Keys._
 import org.scalajs.sbtplugin.cross.CrossProject
 
+crossScalaVersions in ThisBuild := Seq("2.11.11", "2.10.6", "2.12.2", "2.13.0-M1")
+scalaVersion in ThisBuild := (crossScalaVersions in ThisBuild).value.head
+
 val commonSettings: Seq[Setting[_]] = Seq(
   version := "0.2.2-SNAPSHOT",
   organization := "org.scala-js",
-  scalaVersion := "2.11.8",
   scalacOptions ++= Seq("-deprecation", "-feature", "-Xfatal-warnings"),
 
   homepage := Some(url(http://github.com/%3Cspan%20class=%22pl-s%22%3E%3Cspan%20class=%22pl-pds%22%3E"%3C/span%3Ehttp://scala-js.org/%3Cspan%20class=%22pl-pds%22%3E"%3C/span%3E%3C/span%3E)),