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

Closes #157

Author: greenkeeper[bot]

package.json

@@ -47,7 +47,7 @@
     "@types/core-js": "0.9.43",
     "canonical-path": "0.0.2",
     "concurrently": "^3.5.0",
-    "core-js": "^2.4.1",
+    "core-js": "^3.1.3",
     "http-server": "^0.11.0",
     "jasmine-core": "~2.8.0",
     "karma": "^2.0.0",