Doesn't seems to work on Owncloud>=8.1

Since I upgraded my owncloud, I get the following error:

```
CRITICAL:root:While syncing account "cloud" an error occured:
  501 Server Error: Not Implemented for url: https://cloud.*****.net/remote.php/carddav/addressbooks/maethor/default/
```

This could be related to https://github.com/owncloud/contacts/issues/939

Thanks.
