Fix country code being incorrectly passed to phony

In `PhonyPlausibleValidator`.

You cannot pass country code (like 'NL') to `Phony.plausible?` as it
allows only country numbers (like '31'):

    Phony.plausible?('+1 555 1234 123', cc: 'US')
    => false

    Phony.plausible?('+1 555 1234 123', cc: '1')
    => true

Also make a strong distinction between country codes and country numbers
and don't use one when you mean the other.

Fixes #48.