upgrade to latest Scala.js 0.6.x

Author: SethTisue

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "1.0.13")
 
-addSbtPlugin("org.scala-js"     % "sbt-scalajs"              % "0.6.18")
+addSbtPlugin("org.scala-js"     % "sbt-scalajs"              % "0.6.20")
 addSbtPlugin("org.scala-native" % "sbt-crossproject"         % "0.2.0")
 addSbtPlugin("org.scala-native" % "sbt-scalajs-crossproject" % "0.2.0")
 addSbtPlugin("org.scala-native" % "sbt-scala-native"         % "0.3.1")