chore(package): update webpack to version 4.18.0

Closes #190

Author: greenkeeper[bot]

package.json

@@ -80,7 +80,7 @@
     "rollup-plugin-postcss": "1.6.2",
     "rollup-plugin-url": "1.4.0",
     "semantic-release": "^15.5.0",
-    "webpack": "^3.8.1"
+    "webpack": "^4.18.0"
   },
   "files": [
     "dist"