Switch to mainline of SBT 0.13

Now that https://github.com/sbt/sbt/pull/1092 is merged

Review by @cunei / @jsuereth

/cc @adriaanm, @gkossakowski, to remind us that we're still
[flying blind](https://jenkins-dbuild.typesafe.com:8499/job/Community-2.11.x/42/consoleFull)