Fixes msg referenced before assignment

Should fix this error coming up in the logs when receiving Jenkins notifications with the status "COMPLETED":

    sevabot.frontend.api - ERROR - local variable 'msg' referenced before assignment
    Traceback (most recent call last):
      File "/home/skype/sevabot/sevabot/frontend/api.py", line 55, in dispatch_request
        msg = self.compose()
      File "/home/skype/sevabot/sevabot/frontend/api.py", line 189, in compose
        return msg