Ahoy 2.0.0 [1] introduced automatic UUID generation for visit_token and
visitor_token. As a result, the custom ensure_uuid method is no longer
needed and can be safely removed from the initializer.
Since we aren't manually generating UUIDs anymore, we no longer need
the uuidtools dependency.
[1] https://github.com/ankane/ahoy/blob/v2.0.0/README.md#token-generation