chore(package): update webpack to version 4.21.0

Closes #18

Author: greenkeeper[bot]

package.json

@@ -40,7 +40,7 @@
     "mocha": "^3.2.0",
     "nyc": "^11.0.2",
     "prettier": "^1.6.1",
-    "webpack": "^3.1.0"
+    "webpack": "^4.21.0"
   },
   "dependencies": {
     "persistgraphql": "^0.3.5",