Consider moving to Rollup for bundling

[dstaley]

Hello! I've been using my own fork of Kinto that is built using Rollup with great results. Since both Kinto.js and kinto-http.js are written in ES6, Rollup can be used to build ES6-module based bundles as well.

I'd send a PR containing my Rollup config, but I'm still a bit confused about what is necessary for the Firefox-specific build. Switching to Rollup doesn't necessarily mean the Firefox bundle can't still be built with Browserify.

The last change I made in my fork was the removal of @babel/polyfill. With the current config, it's polyfilling everything in the entire polyfill. It was mentioned a while back that it might be better to let developers manually include the polyfills they need based on their target environment. I've been using a polyfill-less build in Chrome over the past year and haven't run into any issues, so I suspect most of the necessary polyfills are already supported in evergreen browsers.

[dstaley]

Closing in favor of #999