Reload order to access proper order data

Fixes #4625
Add Payment#cancel!

This is now called when an order is cancelled, and can do one of two things:

1) It will call the payment method's cancel method if it has one (BraintreeGateway is so far the only gateway to have one)
2) It will call credit! and act like cancelling an order used to

This is part two of the fix for spree/spree_gateway#145