adds codes for curacao and kosovo

pls2review @jduff @camilo 
/cc @DavidGeukers 

Addresses https://github.com/Shopify/shopify/issues/3431 by adding Curaçao and Kosovo to country.rb. Kosovo still does not have ISO-assigned values (although the US-based FIPS org has assigned the two-letter code 'KV'), so I've used the following:
`{ :alpha2 => 'KV', :name => 'Kosovo', :alpha3 => 'KSV', :numeric => '377' }`
Curaçao is ISO-defined:
`{ :alpha2 => 'CW', :name => 'Curaçao', :alpha3 => 'CUW', :numeric => '531' }`