Auto-update demo page on a new tagged release

[Herschel]

It'd be handy to create a GitHub or Circle-CI hook to auto-update the demo page with the latest version whenever there is a tagged release of the emulator on GitHub.

[Herschel]

Currently we use the nightly build on Ruffle CI. This is fine for now as development is rapid -- when we start having stable releases, it might be better to sticky it to a stable release.