chore(package): update core-js to version 3.3.1

Closes #65

Author: greenkeeper[bot]

package.json

@@ -58,7 +58,7 @@
     "codelyzer": "4.5.0",
     "commitplease": "3.2.0",
     "conventional-github-releaser": "^3.1.2",
-    "core-js": "2.6.5",
+    "core-js": "3.3.1",
     "css-loader": "2.1.1",
     "cssnano": "4.1.10",
     "del": "4.0.0",