fix(package): update core-js to version 3.1.3

Closes #20

Author: greenkeeper[bot]

package.json

@@ -18,7 +18,7 @@
     "babel-loader": "^8.0.6",
     "browser-sync": "^2.26.5",
     "browser-sync-webpack-plugin": "^2.2.2",
-    "core-js": "^2.6.3",
+    "core-js": "^3.1.3",
     "cross-spawn": "^6.0.5",
     "css-loader": "^2.1.0",
     "file-loader": "^3.0.1",