PhonyRails.country_number_for should accept case agnostic country code

I came across an issue with lowercase country code data (using Factual's Global Location data). Because I had lowercase country codes, `PhonyRails.country_number_for` was coming back `nil`, since the `ISO3166::Country::Data` hash uses capital letters as keys.

I'm assuming those who may use a lowercase country code e.g. "nl" intend "NL", and so I adjusted the `PhonyRails.country_number_for` method as such.

All tests are passing. Let me know if you're in agreement!

Cheers,
Aron