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

Closes #77

Author: greenkeeper[bot]

package.json

@@ -85,7 +85,7 @@
     "codecov": "^2.2.0",
     "codelyzer": "^3.0.1",
     "concurrently": "^3.4.0",
-    "core-js": "^2.4.1",
+    "core-js": "^3.4.8",
     "execa": "^0.7.0",
     "glob": "^7.1.1",
     "jasmine-core": "^2.5.2",