field_currency_name fix.

Look at the error messages on [Travis](https://travis-ci.org/RubyMoney/money-rails/jobs/43583465):

   NoMethodError:
         undefined method `gbp' for #<Product:0x00000004a2ee10>

This is coming from the line `Money::Currency.find(send(field_currency_name))`, because `send(field_currency_name)` sends the message `'gbp'` to the product, which the product doesn't know how to handle.

Maybe you want to write "send(:field_currency_name)" (notice the extra `:`), but if you're going to do that then there's no point using `send`, you can just use `field_currency_name` on its own.