Merge pull request #88 from scala-steward/update/sbt-scalajs-1.6.0

Update sbt-scalajs, scalajs-compiler, ... to 1.6.0

Author: xuwei-k

project/plugins.sbt

@@ -1,3 +1,3 @@
 addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "2.2.4")
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.5.1")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.6.0")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.0.0")