Add support for Scala 2.11, drop support for 2.9

I'm not quite sure if sbt needs to be bumped, but I thought it couldn't hurt. (Famous last words.)

Drops support for 2.9 because the bumped unfiltered dependencies aren't published for 2.9.x. I'm guessing it should be OK since the argonaut-unfiltered version was already bumped to 6.1-SNAPSHOT.