Rollup build

[inorganik]

This PR doesn't change the countUp.js code at all, just changes the build to only use rollup, and drop gulp. Gulp has not been maintained for a long time, and was being used to concatenate the polyfill with countUp, but with this PR the polyfill will be offered separately. So the dist folder will have:

dist/
├─ countUp.d.ts
├─ countUp.js
├─ countUp.min.js
├─ countUp.umd.js
├─ requestAnimationFrame.polyfill.js