Don't include SnailHelpers if using Carmen-Rails gem

Snail's implementation of `region_options_for_select` conflicts with a much more useful implementation in the widely-used [carmen-rails](https://github.com/jim/carmen-rails) gem.  This change will check first to make sure the `Carmen::Rails` module is not defined before including the Snail helper that defines the conflicting version of `region_options_for_select`.

P.S. - I'm not really sure why Snail needs to implement `region_options_for_select` in the first place.  It seems outside of the scope of the Snail gem, which is for pretty-formatting addresses, not for editing them.  My preferred fix would be to just eliminate Snail's `region_options_for_select`, but perhaps somebody out there is using it...