Downgrade webpack-dev-server pending webpack4 support.

Fixes issue #336, webpack-dev-server exits with an error due to changes
in the webpack api.

{{cookiecutter.app_name}}/package.json

@@ -46,7 +46,7 @@
     "style-loader": "^0.23.0",
     "url-loader": "^1.0.1",
     "webpack": "^2.6.1",
-    "webpack-dev-server": "^3.1.5",
+    "webpack-dev-server": "^2.11.1",
     "sync-exec": "^0.6.2"
   }
 }