code: def release_ids(self):
          
          r = [(k, self._releases[k][0]['upload_time'])
               for k in self._releases.keys()
               if len(self._releases[k]) > 0]
          return [k[0] for k in sorted(r, key=lambda k: k[1])]
docs: >>> package = yarg.get('yarg')
              >>> package.release_ids
              [u'0.0.1', u'0.0.5', u'0.1.0']