Switch to Payment.create to fix issues with payment_instrument_id on payments

@artfulrobot This fixes #63 (closed). I started debugging and then thought what happens if I switch to the new preferred api Payment.create instead of completetransaction. And by magic it works perfectly.

Payment.create calls completetransaction internally if required. As this is now our preferred, tested route for completing a transaction it makes sense to switch rather than investigate a broken "legacy" code-path.