iso_urls_update.rb: fix broken web scraping
iso_urls_update.rb: fix broken web scraping

The web site put in a newline that breaks the assumption of the regex parse.

This commit switches the scraper from regex parsing to XML parsing, which
hopefully is more robust.

Taken from the submission by @shajra-simspace in:

 - https://github.com/nix-community/nixbox/issues/59
 - https://github.com/shajra-simspace

zimbatm/nixbox,zimbatm/nixbox