When using Scala.js 0.6.20 (`addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.20")` in `project/plugins.sbt`), the following error happens when trying to sbt import the project using sbt 0.13.15:

```
java.lang.NoSuchMethodError: sbt.package$.Zero()Lsbt/Global$;
        at org.scalajs.sbtplugin.ScalaJSPluginInternal$.org$scalajs$sbtplugin$ScalaJSPluginInternal$$scalaJSStageSettings(ScalaJSPluginInternal.scala:254)
        at org.scalajs.sbtplugin.ScalaJSPluginInternal$$anonfun$22.apply(ScalaJSPluginInternal.scala:446)
        at org.scalajs.sbtplugin.ScalaJSPluginInternal$$anonfun$22.apply(ScalaJSPluginInternal.scala:446)
        at scala.Function2$$anonfun$tupled$1.apply(Function2.scala:54)
        at scala.Function2$$anonfun$tupled$1.apply(Function2.scala:53)

```

Similar problem: <https://gitter.im/scala-js/scala-js?at=59b53890b59d55b823db1dcd>