don't assume activerecord ids are integers

Hey. So I had a bunch of errors appear when I started trying to ```update_index``` that was backed by an ActiveRecord model with a uuid primary key. It turns out the ```Chewy::Type::Adapter::ActiveRecord``` class is forcing all ids to be integers before it imports them. I've taken that out, and all the specs continue to pass. Also, my indexing works again.

What was the reason for this ```to_i```?