Fix logout links

## Fixes logout links for 2-2-stable.

`spree.destroy_spree_user_session_path` throws a 404 in 2-2-stable.  This update fixes that issue, relying on the underlying devise routes instead.  Follows the fix from https://github.com/spree/spree_fancy/commit/894a475ff5a9c635a16b65fe01ed9f47575681d7.