chore(package): update webpack to version 3.9.0

Author: greenkeeper[bot]

package.json

@@ -34,7 +34,7 @@
     "istanbul": "^0.4.5",
     "mocha": "4.0.1",
     "nyc": "^11.3.0",
-    "webpack": "3.8.1",
+    "webpack": "3.9.0",
     "yargs": "10.0.3"
   },
   "repository": {