Replaces Nokogiri with Oga

As it turns out, replacing it was pretty easy. Was surprised to find no specs breaking in the absence of Nokogiri, while Oga hadn't been added and no code had already been changed. In other words: we need tests :-)

Also noticed that we're parsing the HTML page for releases here... I'm wondering if we're not better off using the GENERIC url that debsecan uses.
Hound, are you on drugs? :-)