chore(package): update webpack to version 4.28.0

Closes #34

Author: greenkeeper[bot]

package.json

@@ -12,7 +12,7 @@
     "coveralls": "2.x",
     "nyc": "^11.0.3",
     "standard": "^10.0.2",
-    "webpack": "^3.0.0"
+    "webpack": "^4.28.0"
   },
   "engines": {
     "node": ">= 6.0.0"