Remove redundant countries prefix on config keys

[lupinitylabs]

Using https://github.com/antonioribeiro/countries-laravel, there is no way to enable or disable caching by overwriting the countries.php config. I found that this is due to redundant countries. prefixes in the Cache Service and Nette classes which lead to effectively reading from countries.countries.cache.enabled, thus ignoring any settings for cache.enabled.

This is reproducible by publishing the config and changing any of the cache settings, for example the temp directory. All of the settings will be ignored, because the request to countries.countries.cache.enabled will return null, which is cast to false when returned from the Cache's enabled() method.

This PR will remove the redundant countries prefixes and will presumably also fix antonioribeiro/countries-laravel#17.

[lupinitylabs]

The original code does not seem to be formatted to match the StyleCI settings.
Do you want me to change and commit the doc block formatting accordingly?