Updated Atom.xml to use correct namespace (#105)
Updated Atom.xml to use correct namespace (#105)

As per https://tools.ietf.org/html/rfc4287#section-1.2, the correct xml namespace is "http://www.w3.org/2005/Atom", which some feed validators will validate against, even though the "https" version of that URL is also valid.  

fixes #104
hexojs/hexo-generator-feed