Replace deprecated @babel/polyfill package with core-js

As of Babel 7.4.0, the `@babel/polyfill` package has been deprecated in
favor of directly including `core-js/stable` and
`regenerator-runtime/runtime` (1). Therefore the package has been
replaced with the `core-js` (2) package.
Currently only major version
2 is supported due to a transitive dependency conflict causing Gatsby
to error out when using `core-js@3` or higher!

References:
  (1) https://babeljs.io/docs/en/next/babel-polyfill.html
  (2) https://www.npmjs.com/package/core-js

GH-137

package.json

@@ -90,11 +90,11 @@
     "webpack-bundle-analyzer": "3.3.2"
   },
   "dependencies": {
-    "@babel/polyfill": "7.4.3",
     "arctic-landscape": ">=0.1.0 <1.0.0",
     "arctic-ocean-fractal": ">=0.1.0 <1.0.0",
     "axios": "0.18.0",
     "body-scroll-lock": "2.6.1",
+    "core-js": "2.6.5",
     "date-fns": "2.0.0-alpha.27",
     "gatsby": "2.3.29",
     "gatsby-image": "2.0.40",