Bump sbt to 1.3.8

[hseeberger]

With sbt 1.3.8 this plugin now works for me. Please consider creating another RC.

[jatcwang]

Thanks @hseeberger. CI is failing - does sbt scripted tests pass for you locally?

[hseeberger]

Nah ... I did not even run the tests ... but nevertheless the locally published plugin works in my setting ;-)

So, either the tests need to be adjusted or the codebase or – hopefully not – sbt itself. Not enough time to investigate that, though.

[jrudolph]

0.10.0-RC1 should probably work but as you noticed the tests are broken because some aspects are still broken with coursier.