Update matcher protocol in parslet/rig/rspec

"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"

http://myronmars.to/n/dev-blog/2014/02/rspec-2-99-and-3-0-beta-2-have-been-released

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. But maybe that's overkill? For example, [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.