Allow `iteration` attribute to be a string

At least with Debianoids the iteration (or revision) can include alphabetical and even some special characters. For example backported packages often use a formats like `-1~bpo60+1`.

fpm just uses `iteration.to_s` so it doesn't care ([code](https://github.com/jordansissel/fpm/blob/v0.4.36/lib/fpm/package.rb#L328-L339)).