Alias method for Rspec protocol compat.

- Pre-Rspec 1.2 failure message protocol was `failure_message` and `negative_failure_message`
- Rspec 2 protocol was `failure_message_for_should` and `failure_message_for_should_not`
- Rspec 3 protocol is `failure_message` and `failure_message_when_negated`
Merge pull request #5 from l15n/rspec3-protocol

Alias method for Rspec protocol compat.