Replace babel-preset-latest with babel-preset-env

[jhwohlgemuth]

While lamenting the spotty support for ES6 in modern tooling, I stumbled upon your article on making a wrapper for fetch in ES6. Thank you for sharing your experiences.

I thought you might be interested in the fact that babel-preset-latest appears to be deprecated in favor of babel-preset-env. The babel docs say that the "env" preset with no options acts just like the "latest" preset.

Feel free to do whatever you want with this PR. I just wanted to try and give you a little help keeping one of your projects up-to-date 😉👍

Note: The transpiled output is different, but it might actually be better. Full disclosure, I did not test the newly transpiled code...