Return countries with two alpha characters in maxmind geocoder

Return countries as two alpha characters on maxmind geocoder as the same as other geocoders:

This is the current behaviour:

```
irb(main):001:0> Geokit::Geocoders::IpGeocoder.geocode('8.8.8.8').country_code
=> "US"
irb(main):002:0> Geokit::Geocoders::RipeGeocoder.geocode('8.8.8.8').country_code
=> "US"
irb(main):003:0> Geokit::Geocoders::MaxmindGeocoder.geocode('8.8.8.8').country_code
=> "USA"
``