Add Laravel 5.6 support

[padarom]

For reasons see #6

mledoze/countries no longer contains countries with empty regions, they are all using the new region Antarctic now. As such I removed \Countries::$REGION_NONE and added \Countries::$REGION_ANTARCTIC.

Also, the subregion Northern America was renamed to North America. I updated the tests to reflect that.

Both of these changes are compatibility breaking, so this should be a new major version.

[coveralls]

Coverage decreased (-0.5%) to 92.683% when pulling 80c826f on padarom:laravel56 into 3f7f388 on Lykegenes:master.

[padarom]

Build fails for PHP 5.6 and PHP 7.0

Laravel 5.6 upped the minimum version to PHP7.1 so I had to change the dev dependencies to reflect that.