field_currency_name fix.

This line doesn't make sense.

Think of it broken down like this:

    a = field_currency_name
    b = send(a)
    c = Money::Currency.find(b)

The second line is going to fail because it's not looking for a method called `field_currency_name`, it's looking for the *result* of `field_currency_name` - in other words it's looking for a method called e.g. `usd` or `gbp` which (probably) doesn't exist.