Bump to Scala.js 0.6.0-M3

The resolver is not needed anymore as Scala.js is pushed to
Maven Central.

See also http://www.scala-js.org/news/2014/12/22/announcing-scalajs-0.6.0-M3/
Merge pull request #54 from tindzk/scalajs0.6.0-m3

Bump to Scala.js 0.6.0-M3