Updating rspec_matchers for Rspec 2 or 3

See [rspec-expectations gem v2.99.0 tag, lib/rspec-expectations.rb](https://github.com/rspec/rspec-expectations/blob/v2.99.0/lib/rspec-expectations.rb#L3-L6).

Closes mbklein/equivalent-xml#17
Merge pull request #18 from jeremyf/updating-rspec-for-rspec-3

Updating rspec_matchers for Rspec 2 or 3