Use NONET when check-mirrors parses through Nokogiri
Originally created by @sajolida on #8537 (Redmine)
This is recommended for untrusted documents:
http://www.nokogiri.org/tutorials/parsing_an_html_xml_document.html#parse_options
https://github.com/sparklemotion/nokogiri/issues/693#issuecomment-68334768
Feature Branch: check-mirrors:bugfix/8537-safer-nokogiri-parse-options