Disable use of ttml subs

- libass does not handle the converted ttml rolling subs well
- vtt subs are left aligned but position can be overriden in Kodi #489

resources/lib/youtube_plugin/youtube/helper/subtitles.py

@@ -129,7 +129,7 @@ def get_subtitles(self):
             return None
 
         if languages == self.LANG_ALL:
-            return self.get_all(sub_format='ttml')
+            return self.get_all(sub_format='vtt')
 
         if languages == self.LANG_PROMPT:
             return self._prompt()