get_series to use base for AppFacade >= 15

Author: cderici

juju/application.py

@@ -368,7 +368,12 @@ async def get_series(self):
         log.debug(
             'Getting series for %s', self.name)
 
-        return (await app_facade.Get(application=self.name)).series
+        appGetResults = (await app_facade.Get(application=self.name))
+        if self._facade_version() >= 15:
+            base_channel = appGetResults.base.channel
+            track = Channel.parse(base_channel).track
+            return utils.get_version_series(track)
+        return appGetResults.series
 
     async def get_config(self):
         """Return the configuration settings dict for this application.