Minimum PHP version in composer.json is incorrect

[Rowan Tommins]

Currently, the composer.json lists a minimum PHP version of 5.6, but the actual code includes many features which require later versions, such as a type declaration of ?string, which is only valid since PHP 7.1

Either the documented minimum version should be updated, or the code changed to run on older versions.

[Rowan Tommins]

By the way, thanks for making this repo, and I love the use of Basque as a test locale!

[Fernando Herrero]

Thanks a lot