Add cross-building support with Scala.js 1.0.0-M1.

Author: sjrd

.travis.yml

@@ -13,3 +13,6 @@ scala:
   - 2.13.0-M1
 jdk:
   - oraclejdk8
+env:
+  - SCALAJS_VERSION=0.6.18
+  - SCALAJS_VERSION=1.0.0-M1

project/build.sbt

@@ -1,3 +1,6 @@
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.18")
+val scalaJSVersion =
+  Option(System.getenv("SCALAJS_VERSION")).getOrElse("0.6.18")
+
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % scalaJSVersion)
 
 addSbtPlugin("org.scalastyle" % "scalastyle-sbt-plugin" % "0.8.0")