Downgrade back to sbt 0.13.0.

This reverts commit 8f2169a37e77bca4cee8300c638e15d9eca138aa.
This reverts commit eede37cc4fb87db96eae28a9a536c507233f1dd1.
Merge pull request #1071 from sjrd/downgrade-sbt

Downgrade back to sbt 0.13.0.