Support for Scala 2.11

Two changes:

  1. Modify build definition to support (not crash) when Scala 2.11 is being used
  2. Upgrade to sbt 0.13.x which allows us to plug spray-json into our [community build](https://github.com/typesafehub/community-builds) which we rebuild every night now

I'd appreciate speedy review of this PR.