Update ruby-rspec-rails to 2.9.0.

Update ruby-rspec-rails to 2.9.0.

Remove restriction for version of Ruby on Rails.

### 2.9.0 / 2012-03-17
[full changelog](http://github.com/rspec/rspec-rails/compare/v2.8.1...v2.9.0)

Enhancments
* add description method to RouteToMatcher (John Wulff)
* Run "db:test:clone_structure" instead of "db:test:prepare" if Active Record's
  schema format is ":sql". (Andrey Voronkov)

Bug fixes
* mock_model(XXX).as_null_object.unknown_method returns self again
* Generated view specs use different IDs for each attribute.