BUGFIX: this allows omnipay to work for currencies other than only USD Paypal (e.g. NZD Paypal).

Amount and currency always come in a pair when you work with Money.  We are adding currency here with amount to prevent Paypal from throwing an error when submitting a non-USD payment.
Merge pull request #55 from sunnysideup/patch-1

FIX: add currency to amount when completing purchase.