fix(package): update core-js to version 3.4.8

Closes #65

Author: greenkeeper[bot]

demo/package.json

@@ -31,7 +31,7 @@
     "@ng-bootstrap/ng-bootstrap": "4.1.0",
     "@stackblitz/sdk": "^1.2.0",
     "bootstrap": "4.3.1",
-    "core-js": "^2.5.4",
+    "core-js": "^3.4.8",
     "font-awesome": "^4.7.0",
     "hammerjs": "^2.0.8",
     "rxjs": "^6.4.0",