downgrade webpack-dev-server (webpack/webpack-dev-server#1101)

Author: jochenberger

package.json

@@ -91,7 +91,7 @@
     "style-loader": "0.19.0",
     "url-loader": "0.6.2",
     "webpack": "3.8.1",
-    "webpack-dev-server": "2.9.4",
+    "webpack-dev-server": "2.7.1",
     "yargs": "10.0.3"
   }
 }