Update sbt-scalajs, scalajs-compiler to 0.6.31 (#249)

Update sbt-scalajs, scalajs-compiler to 0.6.31

Author: lrytz

.travis.yml

@@ -7,7 +7,7 @@ scala:
 
 env:
   - ADOPTOPENJDK=8     SCALAJS_VERSION=
-  - ADOPTOPENJDK=8     SCALAJS_VERSION=0.6.29
+  - ADOPTOPENJDK=8     SCALAJS_VERSION=0.6.31
   - ADOPTOPENJDK=8     SCALAJS_VERSION=1.0.0-M8
   - ADOPTOPENJDK=11    SCALAJS_VERSION=
 

project/plugins.sbt

@@ -1,5 +1,5 @@
 val scalaJSVersion =
-  Option(System.getenv("SCALAJS_VERSION")).filter(_.nonEmpty).getOrElse("0.6.29")
+  Option(System.getenv("SCALAJS_VERSION")).filter(_.nonEmpty).getOrElse("0.6.31")
 
 val scalaNativeVersion =
   Option(System.getenv("SCALANATIVE_VERSION")).filter(_.nonEmpty).getOrElse("0.3.9")