[pin] Use CreditCard#name

Within Spree, we have a field called "Name on Card", which gets assigned to our CreditCard object as `name`. Many gateways within ActiveMerchant already use the `name` method to represent this, but Pin did it differently using `first_name` and `last_name`. Spree does not have a `first_name` and `last_name` field, and so Pin chokes because we're giving it `name`, rather than `first_name` and `last_name`.

This patch aims to standardise Pin into using the same method that many other gateways within ActiveMerchant use.