Use Scala.js specicifed in environment variable.

Author: exoego

project/plugins.sbt

@@ -1,5 +1,7 @@
 scalacOptions ++= Seq("-deprecation", "-feature", "-Xfatal-warnings")
 
+val scalajsVersion = Option(System.getenv("SCALAJS_VERSION")).filter(_.nonEmpty).getOrElse("0.6.28")
+
 addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "2.0.0")
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.28")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % scalajsVersion)
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "0.6.1")