Changing order.token to order.guest_token in accordance with the name change in spree 2.3

According to the [release notes](https://github.com/spree/spree/releases) for spree 2.3, the `Spree::Order` method `token` was changed to `guest_token`, making this gem incompatible with spree 2.3 and up. This fix rectifies that problem.

Also note that this is my first pull request, so please let me know if I've missed something.