Fix swipe.js link

Hotlinked raw.github.com .js files are served with a `text/plain` encoding (I'm not sure why, but maybe because it's not served from a `*.github.io` subdomain?)

As a result, the website does not currently work on Chrome browsers (not tested on others).

This fix uses the [rawgit.com](http://rawgit.com/) service that serves as a CDN and adds the proper Content-Type.