Bump dotty version to get typing improvements

Author: julienrf

build.sbt

@@ -1,6 +1,6 @@
 // Convenient setting that allows writing `set scalaVersion := dotty.value` in sbt shell to switch from Scala to Dotty
 val dotty = settingKey[String]("dotty version")
-dotty in ThisBuild := "0.2.0-RC1"
+dotty in ThisBuild := "0.3.0-bin-20170721-d7faafa-NIGHTLY"
 
 val commonSettings = Seq(
   organization := "ch.epfl.scala",