iATS: Fix invalid country code UK -> GB

Fixes erroneous country code. United Kingdom is really `GB`, not `UK`. Tricky!

@ntalbott I'd like to add this supported country code test to the gateway generator to avoid this in the future:

```ruby
def test_supported_countries
  @gateway.supported_countries.each do |country_code|
  assert ActiveMerchant::Country.find(country_code), "Supported country code #{country_code} is invalid. Please use a value explicitly listed in active_utils' ActiveMerchant::Country class."
  end
end
```

If you think this is sane let me know and I'll add a separate PR.