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?
Ugh... it looks like the block is being `instance_eval`'ed.

Try this:

```
_enum_attribute = enum_attribute

define_model :record_with_array_values do
  enum(_enum_attribute => %w(published unpublished draft)
end
```