Bump Dotty version

This version of Dotty fixes several unsoundness issues and remove the
unsound protected[this] escape hatch for variance checking. See
scala/scala3#4013

Author: allanrenucci

.travis.yml

@@ -1,7 +1,7 @@
 language: scala
 jdk: oraclejdk8
 script:
-  - sbt ++2.12.4 memoryBenchmark/compile test ++2.13.0-M2 timeBenchmark/compile test ++0.7.0-RC1 test
+  - sbt ++2.12.4 memoryBenchmark/compile test ++2.13.0-M2 timeBenchmark/compile test ++0.8.0-bin-20180317-b499a9a-NIGHTLY test
   - sbt ++2.13.0-M2 collections-contribJVM/test junit/test scalacheck/test
   - sbt ++2.12.4 collectionsJS/test ++2.13.0-M2 collectionsJS/test collections-contribJS/test
 before_script:

build.sbt

@@ -3,7 +3,7 @@ import org.scalajs.sbtplugin.cross.CrossProject
 
 // 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 := "0.7.0-RC1"
+dotty in ThisBuild := "0.8.0-bin-20180317-b499a9a-NIGHTLY"
 
 val collectionsScalaVersionSettings = Seq(
   scalaVersion := "2.13.0-M2",