Remove this dependency so that the gem can be used with any ruby project, including Rails 2.x.
You may find aequitas useful for validations to replace ActiveModel's. It has integration with Virtus.
Using aequitas or any other validation library is shouldn't be a hard dependency. For those who do already use activemodel/support, being able to reuse that functionality would be good instead of pulling in a library with similar-but-not-the-same features.
Agree wrt it not being a hard dependency; in fact I'm not sure I see any reason to have it integrated into the gem at all, other than a mention in the README.