fix(package): update core-js to version 3.3.3

Closes #44

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "chai": "^4.1.2",
     "chai-http": "^3.0.0",
     "cookie-parser": "latest",
-    "core-js": "^2.5.3",
+    "core-js": "^3.3.3",
     "cors": "^2.8.4",
     "dotenv": "^5.0.1",
     "errorhandler": "latest",