Two minor comments:

• Could you extract a variable for $scriptsDir/repositories-scala-release and refer to it in sbtArgs=? That will make the data dependencies between the steps in the script more obvious.
• Given that this is pretty esotoric SBT config, add a link the relevant SBT docs: http://www.scala-sbt.org/0.13/docs/Proxy-Repositories.html and explain the intent ("PR valiadation uses a dedicated repostory to share artifcacts between jobs, so we need to configure SBT to use these rather than its default, Maven Central")

Otherwise LGTM. Great to see we'll be able to handle the bintray switch a relatively low cost.

Amended.