Clean build.sbt a little

First off, it seems that lines https://github.com/typelevel/wartremover/blob/d6b95c3a3720be45aff2d4093256671dd2ebf8ba/build.sbt#L54-L59 are not used anymore.

Fixed imports to be more explicit (previous were `red code' in IntelliJ).

Also changed scalac version to 2.11.1 (2.11.0 should not be used, although its bug does not touch WR) and updated scm links in `pomExtra`. =)