chore(package): update webpack to version 4.15.0

Closes #34

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "phantomjs": "^2.1.7",
     "serve": "^9.0.0",
     "standard": "^10.0.2",
-    "webpack": "^3.3.0"
+    "webpack": "^4.15.0"
   },
   "scripts": {
     "test": "npm run lint && karma start karma.conf.js",