Add 65 additional countries that do not require postcodes

This adds 65 countries, aside from Ireland, that do not use postcodes. As well, it adds a class method for telling if a country's postcode is required, i.e.
```ruby
GoingPostal::Postcode.not_required?('IE')
# => true
```
And the converse:
```ruby
GoingPostal::Postcode.required?('IE')
# => false
```