Only update timezone on address change

Your proposed change doesn't match the existing logic. I usually avoid using `unless` with || for that reason. 

```ruby
return unless !primary_mailing_address?
            || (changed & %w(street city state country primary_mailing_address)).blank?
            || !addressable.respond_to?(:set_timezone)
addressable.set_timezone
```
`unless !` is always a stumbling block