chore(package): update core-js to version 3.4.6

Closes #15

Author: greenkeeper[bot]

package.json

@@ -42,7 +42,7 @@
     "alsatian": "^2.2.1",
     "alsatian-fluent-assertions": "^1.0.0-alpha018",
     "codecov": "^3.0.0",
-    "core-js": "^2.5.6",
+    "core-js": "^3.4.6",
     "cross-env": "^5.1.5",
     "npm-run-all": "^4.1.3",
     "nyc": "^12.0.1",