fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ASYNC-7414156

Author: snyk-bot

package.json

@@ -45,15 +45,15 @@
     "extract-text-webpack-plugin": "1.0.1",
     "file-loader": "0.9.0",
     "fs-extra": "^0.30.0",
-    "html-webpack-plugin": "2.22.0",
+    "html-webpack-plugin": "4.0.0",
     "json-loader": "0.5.4",
     "opn": "4.0.2",
     "postcss-loader": "0.9.1",
     "rimraf": "2.5.3",
     "semver": "^5.3.0",
     "style-loader": "0.13.1",
     "url-loader": "0.5.7",
-    "webpack": "1.13.1",
+    "webpack": "4.0.0",
     "webpack-dev-server": "1.14.1"
   },
   "devDependencies": {