Update to work with the current site

* Stop using the RSS feed, which never gets updated, and use the JSON
  file that JS on the site uses.
* Update the method for fetching file download URLs, since the old one
  is broken.
* Use shutil to download the file instead of a manual loop. Also
  increase the buffer size to 1MB.
Change indents from 2 spaces to 4 spaces

I must have been in a Ruby mood originally.