Fixed get_default_role in Server attempting to access the wrong roles.

discord/server.py

@@ -167,6 +167,6 @@ class Server(object):
 
     def get_default_role(self):
         """Gets the @everyone role that all members have by default."""
-        for role in roles:
+        for role in self.roles:
             if role.name == '@everyone':
                 return role