chore(package): update webpack to version 4.28.0

Closes #32

Author: greenkeeper[bot]

package.json

@@ -22,7 +22,7 @@
     "source-loader": "^1.0.0",
     "standard": "^11.0.1",
     "sugarml": "^0.6.0",
-    "webpack": "^3.3.0",
+    "webpack": "^4.28.0",
     "when": "^3.7.8"
   },
   "engine": ">=6",