Update sbt-scalajs, scalajs-compiler to 0.6.31 (#133)

Author: scala-steward

project/plugins.sbt

@@ -1,5 +1,5 @@
 addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.4.31")
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.29")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.31")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "0.6.1")
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.1")
 addSbtPlugin("io.get-coursier" % "sbt-coursier" % "2.0.0-RC5-1")