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 this and the next line, what about saying the following:

```
desc = "define #{attribute_name} as an enum"
```

Then the test would read

> Post should define :attribute as an enum

and the failure message would read

> Expected Post to define :attribute as an enum