use latest Scala.js

this is required to fix Travis failures after scala#109 was merged

Author: SethTisue

project/plugins.sbt

@@ -1,4 +1,3 @@
 addSbtPlugin("org.scala-lang.modules" % "scala-module-plugin" % "1.0.4")
 
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.12")
-
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.14")