Fixes #84

This pull request changes the order details deface. Previously the file was still referring to `order.payment` in two places, even after https://github.com/spree/spree_paypal_express/commit/020d519357d0c68bb11acd525ba2a2f8c316dc22
This fix isn't particularly graceful, as I'm new to ruby, but it does the trick; i.e. after paypal redirects back to my spree store, it doesn't show an error page saying that it can't find the order.payment method.