Use Scala.js 0.2.
Use Scala.js 0.2.1.
Remove resolver in project/build.sbt.

The Scala.js sbt plugin now in the standard repository for
sbt plugins, and can therefore be resolved by default.
Upgrade to Scala.js v0.3.
Remove obsolete .gitignore.
Upgrade to Scala.js v0.4-SNAPSHOT and its breaking changes.