ActiveRecord::Enum matcher

Hi guys,

I started a matcher for the  [ActiveRecord::Enum](http://edgeapi.rubyonrails.org/classes/ActiveRecord/Enum.html). I know we can`t merge this now since no released rails 4.1.0.

I only want to show you guys the idea =) and ask one thing: ActiveRecord::Enum is only on rails 4.1.0, how we can skip this specs in older rails versions?
Instead of having this method, would it be a good idea to rename `@subject` to `@model`, and define a protected attr_reader for it? This way `expectation` would read:

```
def expectation
  "#{model.class.name} to #{description}"
end
```