fix(package): update core-js to version 3.1.3

Closes #77

Author: greenkeeper[bot]

demo/package.json

@@ -43,7 +43,7 @@
     "@angular/service-worker": "^7.2.10",
     "@ng-bootstrap/ng-bootstrap": "^4.1.0",
     "bootstrap": "^4.3.1",
-    "core-js": "^2.6.5",
+    "core-js": "^3.1.3",
     "font-awesome": "^4.7.0",
     "include-media": "^1.4.9",
     "material-design-icons": "^3.0.1",