Merge pull request #1757 from mozilla/greenkeeper/webpack-dev-server-2.3.0

chore(package): update webpack-dev-server to version 2.3.0

package.json

@@ -260,7 +260,7 @@
     "tosource": "1.0.0",
     "webpack": "1.14.0",
     "webpack-dev-middleware": "1.10.0",
-    "webpack-dev-server": "1.16.2",
+    "webpack-dev-server": "2.3.0",
     "webpack-hot-middleware": "2.15.0"
   }
 }