chore(package): update core-js to version 3.4.0

Closes #87

Author: greenkeeper[bot]

package.json

@@ -64,7 +64,7 @@
     "codelyzer": "^4.5.0",
     "commitplease": "^3.2.0",
     "conventional-github-releaser": "2.0.2",
-    "core-js": "^2.6.9",
+    "core-js": "^3.4.0",
     "css-loader": "^2.1.0",
     "cssnano": "^4.1.8",
     "del": "^3.0.0",