update webpack-dev-server to v2.6.1

package.json

@@ -83,6 +83,6 @@
     "karma-sourcemap-loader": "^0.3.7",
     "karma-webpack": "^2.0.4",
     "nodemon": "^1.11.0",
-    "webpack-dev-server": "^2.4.2"
+    "webpack-dev-server": "^2.6.1"
   }
 }