Add babel-polyfill as a dependency

[sdellis]

This is always required for this to work in the browser, so might as well make it a dependency. I would add the issue, but my repo is a fork of yours @jpstroop . Will resolve later, but just want to leave this here so I don't forget.

[jpstroop]

The README has this note. I think you should feel free to develop your fork as the primary version. LMK if there's anything you want me to do.