Starting with versions > 0.3.0 activemodel has been listed as a dependency. It tries to pull down activesupport > 3.0.0 which generally makes this gem incompatible with Rails 2. Is there any way to skip listing that dependency in the gemspec?
I've went through the gem and it looks like removing activemodel at this stage would be really involved. It seems all of the validation code is now piggy-backing off of activemodel. It might just be worth mentioning in the README that this gem is no longer compatible with Rails 2 since version 0.3.0.