Add support for the latest RSpec matcher protocol

This patch fixes the deprecation warnings given when using
email-spec with RSpec 3.0.0.beta2. The negative_failure_message
method has been renamed to failure_message_when_negated and
an alias for negative_failure_message has been added to support
older RSpec releases.
Merge pull request #150 from yabawock/rspec3

Add support for the latest RSpec matcher protocol