ActiveModel::Dirty attribute methods don't work with older ActiveModel

[jmoses]

The #{attribute_name}_changed? and similar methods from ActiveModel::Dirty do not work except for the created_at attribute if you're using a version of ActiveModel older that 3.1.0.

I'm not sure if it's realistic to make it do that or not, but it's a bug at least to not require a newer version of ActiveModel.