Fix betaseries warning during post processing

Add notify_subtitle_download() method to Betaseries notifier.

sickbeard/notifiers/betaseries.py

@@ -39,6 +39,9 @@ class BetaSeriesNotifier:
     def notify_download(self, ep_name):
         pass
 
+    def notify_subtitle_download(self, ep_name, lang):
+        pass
+
     def update_library(self, ep_obj):
         """
         Sends a request to betaseries indicating that the given episode is part of our library.