Cross-build to 2.11.0-RC3, cleaner build.sbt

Hi, it’s me again :) Updated to 2.11.0-RC3.
I would be really grateful if you could cross-publish this, as *Wartremover* is the only non-`2.11` dependency in my project (https://github.com/macroid/macroid), and I want to cross-publish the upcoming new version soon.