Update matcher protocol in parslet/rig/rspec

http://myronmars.to/n/dev-blog/2014/02/rspec-2-99-and-3-0-beta-2-have-been-released

> Update matcher protocol and custom matcher DSL to better align with the
newer expect syntax. If you want your matchers to maintain compatibility
with multiple versions of RSpec, you can alias the new names to the old.
(Myron Marston)
> * failure_message_for_should => failure_message
> * failure_message_for_should_not => failure_message_when_negated
> * match_for_should => match
> * match_for_should_not => match_when_negated

Sorry, no tests for RSpec 3 integration. I'm eager to add them but I'm unsure how.

It's possible to check in two gemfiles with different Rspec versions into project and so that Travis runs against both of them. [Example](https://github.com/collectiveidea/json_spec/blob/master/.travis.yml). But maybe that's overkill? It makes tests 2x longer (they're pretty quick, though). [Capybara supports both RSpec versions](https://github.com/jnicklas/capybara/blob/master/lib/capybara/rspec/matchers.rb#L65), but CI checks it against 2.x only.