update countries from active merchant

ActiveMerchant requires active_utils as dependency and both of those gems define ActiveMerchant::Country::COUNTRIES (slightly differently). That's super shitty.

This PR pulls in [country.rb](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/country.rb) from ActiveMerchant so that the file can be removed from there in a follow-up PR (and thus get rid of annoying "already initialized constant ActiveMerchant::Country::COUNTRIES" warnings in every project that uses ActiveMerchant).

I double checked and the COUNTRIES hash is the same in both, just 1.8 vs. 1.9 syntax.

@j-mutter @jnormore @eapache 
no idea, because someone removed it in activemerchant already I guess. All this PR does is copy the newer file from activemerchant over here into this gem.