Can't send a blank state for Skipjack

This is to fix https://github.com/Shopify/shopify/issues/4050

The 'State' field is required by Skipjack ([page 26](http://www.trishabrowncompany.org/content/documents/5Skipjack_Integration_Guide-1.pdf)).  We send the province_code for address[:state] on our end, but we don't have a province_code for some countries.  When someone from one of those countries places an order, the order is rejected by Skipjack because "The value or length for billing state is empty."

[As you can see](http://monosnap.com/image/otLjbnbIrLyvPDKy1XGpOXkAu) on page 221 of their documentation, since it's a required variable, they recommend sending 'XX' as a dummy value for 'State'.

The existing remote tests for this gateway all consistently fail, and I was unable to get them passing.  So unfortunately this PR is test-free.

@melari @jduff