chore(package): update core-js to version 3.1.4

Closes #1080

Author: greenkeeper[bot]

package.json

@@ -73,7 +73,7 @@
     "babel-eslint": "10.0.0",
     "babel-loader": "8.0.6",
     "babel-plugin-transform-object-rest-spread": "6.26.0",
-    "core-js": "2.6.5",
+    "core-js": "3.1.4",
     "css-loader": "2.1.1",
     "eslint": "5.16.0",
     "eslint-plugin-jest": "22.5.1",