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

Closes #34

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "@types/node": "^11.10.0",
     "@types/shelljs": "^0.8.3",
     "ava": "^1.2.1",
-    "core-js": "^2.6.5",
+    "core-js": "^3.4.6",
     "cross-env": "^5.2.0",
     "cross-var": "^1.1.0",
     "husky": "^1.3.1",