Change organization to "org.scala-lang.modules.scalajs".

This was agreed upon as final organization for Scala.js at
the Scala core meeting.

In user projects, you must change your project/build.sbt
(or wherever you define sbt plugins) and update the
organization. So you should have a line with:

addSbtPlugin("org.scala-lang.modules.scalajs" %
    "scalajs-sbt-plugin" % "0.1-SNAPSHOT")