fix(package): update core-js to version 3.1.0

Closes #25

Author: greenkeeper[bot]

package.json

@@ -22,7 +22,7 @@
     "@angular/platform-browser": "^5.2.0",
     "@angular/platform-browser-dynamic": "^5.2.0",
     "@angular/router": "^5.2.0",
-    "core-js": "^2.4.1",
+    "core-js": "^3.1.0",
     "rxjs": "^5.5.6",
     "zone.js": "^0.8.19",
     "bootstrap": "4.0.0"