Unable to pay with credit card when applying a coupon in the payments page

I don't think this is the extension's fault, but rather Spree's.
The problem is that the file `payment.js.coffee` calls `submit()` (unnecessarily) hijacking the flow from braintree and submitting the form before the payment method data can be received.

I'm opening an issue about this in Spree, but I thought it should be referenced here too.
