Cleanup build settings

Since Scalatra 2.4.0 will support only Scala 2.11 and 2.10, we can drop legacy hacks for Scala 2.8, 2.9 deployment. 

There are other points to notice:
- bump sbt to 0.13.7, we don't need blank line anymore
- specs2 2.4.3 or higher requires scalaz bintray repo
- avoid upgrading atmosphere artifacts because I'm not familiar with atmosphere