Make specs compatible with ruby-head

This should address the [failing ruby-head build](https://travis-ci.org/jeremytregunna/ruby-trello/jobs/4112926). Addresses incompatibility between ruby-2.0 and the activemodel dependency, which [calls #initialize_dup as a public method](https://github.com/rails/rails/blob/3-2-stable/activemodel/lib/active_model/validations.rb#L179) (now private in ruby-head).

Rather than mess with dependencies, I opted to rewrite the failing specs so they don't rely on `dup`.