Update Build to Scala 2.11.4

As mentioned by Typesafe: 
> Do Not Use Scala 2.11.3
> Due to a binary incompatibility in Scala 2.11.3, we recommend upgrading to Scala 2.11.4, which resolves the incompatibility, as well as another blocker issue that was discovered in the days after the 2.11.3 release.
cf: http://www.scala-lang.org/news/2.11.4