Use rollup for UMD build

[aweary]

Rollup for libraries, webpack for apps.

[brybrophy]

@boygirl Do you see a need for this one? I think Rollup does output a smaller/more optimized bundle for libraries compared to Webpack, but this component is so small that I'm not sure how much gain we would really see. Webpack has also come a long way since last December. Finally, I haven't seen Rollup being used in any other Formidable projects I have worked on.

Yea or nay? If nay, I think we could close this one.

[boygirl]

I vote nay. I'll close this issue.

[brybrophy]

Good call.