Upgrade to latest version of Rollup

[jonkoops]

Also cleans up the build configuration itself a little so it's a bit more DRY and conforms to the linting standards of the project.

[mourner]

The reason we held on to the older Rollup is that it supported the legacy option, allowing ES3 as the target — we still formally support very old IE versions with Leaflet, and dropping this (although something we should do) would need a major version bump, so needs to be done carefully and with a lot of consideration.

[jonkoops]

@mourner I checked the output between these versions and they seem pretty compatible to me, I will take a look and see if this remains functional in older browsers.

[mourner]

After studying rollup/rollup#2141 very carefully, I discovered that we can keep compatibility with IE8 if we use both freeze: false and esModule: false. 🎉 Haven't actually tested on IE8 (lol), but pretty sure we can proceed with the upgrade.

[mourner]

cc @IvanSanchez

[jonkoops]

Awesome @mourner! Should we also update the supported browsers on the site? It would seem that Internet Explorer 7 isn't supported anymore if I read this correctly.