[WIP] Sign out fixes

1. The HTTP verb for the sign out route has been changed to `GET`. Any requests to `DELETE /g5_auth/users/sign_out` are redirected.
2. Override `after_sign_out_path_for` to return `main_app.root_path`. The default devise implementation for this callback was getting very confused by the fact that it is enclosed in an isolated engine.
3. Upgrade dependency on [devise_g5_authenticatable](https://github.com/G5/devise_g5_authenticatable) to pick up [this fix for signing out when there isn't a user](https://github.com/G5/devise_g5_authenticatable/pull/16).

This is ready for review, but keeping it marked as WIP until [devise_g5_authenticatable](https://github.com/G5/devise_g5_authenticatable/pull/16) is officially released and we can update `g5_authenticatable.gemspec` accordingly.