Adapted to new audio_features response

spotipy/client.py

@@ -633,7 +633,13 @@ class Spotify(object):
                 - tracks - a list of track URIs, URLs or IDs, maximum: 50 ids
         '''
         tlist = [self._get_id('track', t) for t in tracks]
-        return self._get('audio-features?ids=' + ','.join(tlist))
+        results =  self._get('audio-features?ids=' + ','.join(tlist))
+        # the response has changed, look for the new style first, and if
+        # its not there, fallback on the old style
+        if 'audio_attributes' in results:
+            return results['audio_attributes']
+        else:
+            return results
 
     def _get_id(self, type, id):
         fields = id.split(':')