Update ruby-rspec-expectations to 2.9.1.

### 2.9.1 / 2012-04-03
[full changelog](http://github.com/rspec/rspec-expectations/compare/v2.9.0...2.9.1)

Bug fixes

* Provide a helpful message if the diff between two objects is empty.
* Fix bug diffing single strings with multiline strings.
* Fix for error with using custom matchers inside other custom matchers
  (mirasrael)
* Fix using execution context methods in nested DSL matchers (mirasrael)

(taca)