Add support for rspec 3+

Alias new rspec matcher protocol names (rspec 3+) to the old (rspec 2). From https://www.relishapp.com/rspec/rspec-expectations/docs/changelog:

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
Merge pull request #24 from george/master

Add support for rspec 3+