chore(package): update core-js to version 3.1.4

Closes #77

Author: greenkeeper[bot]

package.json

@@ -67,7 +67,7 @@
     "commitplease": "3.2.0",
     "conventional-github-releaser": "2.0.2",
     "copy-webpack-plugin": "^5.0.2",
-    "core-js": "2.5.7",
+    "core-js": "3.1.4",
     "css-loader": "^1.0.0",
     "cssnano": "^4.1.10",
     "del": "3.0.0",