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