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?
One more quick thing. I just merged in support for RSpec 3, so in keeping with other matchers, would you mind changing this to:

``` ruby
def failure_message
  ...
end
alias failure_message_for_should failure_message

def failure_message_when_negated
end
alias failure_message_for_should_not failure_message_when_negated
```