Update the build to sbt 0.13.6

Supersedes #56 
The late-added commit publishes` sbt-0.13.6-on-2.11.3-SNAPSHOT-for-IDE-SNAPSHOT`.
Review by @skyluc, @jsuereth 

preserves scala-pr-validation
@huitseeker, let's try to remove this, and add back the `LOCAL_M2_REPO` before we downgrade. @jsuereth believes this is the reason for validation failures.