upgrade webpack to v3.4.0

package.json

@@ -59,7 +59,7 @@
     "vue-loader": "^11.3.4",
     "vue-resource": "^1.3.4",
     "vue-template-compiler": "^2.2.6",
-    "webpack": "^3.1.0",
+    "webpack": "^3.4.0",
     "webpack-bundle-analyzer": "^2.8.2",
     "webpack-stats-plugin": "^0.1.5"
   },