Add default payment currency to config.php

[andrewkett]

When I installed the newest extension with multi currency I was getting an error on checkout. I was seeing this in our logs:

==> log/magento/payment.log <==
[2018-06-20 11:27:05] main.DEBUG: array (
  'redirect response body' =>
  stdClass::__set_state(array(
     'result' => 'ERROR',
     'error' => 'Specified currency is invalid.',
  )),
) [] []
[2018-06-20 11:27:05] main.DEBUG: array (
  'FAILED TO GET returnURL' =>
  stdClass::__set_state(array(
     'result' => 'ERROR',
     'error' => 'Specified currency is invalid.',
  )),
) [] []

This was because I hadn't gone to admin unchecked "use default" on the payment currency config and saved, therefore there was no database config for payment_currency. This change means that if no config is explicitly set in admin it will default to NZD.