Build improvements = sbt 0.13.5 syntax + upgrades

This is for the meta-project not yours. Since [sbt is recursive](http://www.scala-sbt.org/release/tutorial/Full-Def.html#sbt+is+recursive) there's the meta-project under `project` that drives yours (the main one) and as `addSbtPlugin`s belong to the meta-project, the setting is for it to spit about the features of sbt that are now deprecated.