Update matchers for RSpec 3's matcher protocol

Resolves this deprecation message when using webmock's matchers on a project with RSpec 3:

> WebMock::WebMockMatcher implements a legacy RSpec matcher
> protocol. For the current protocol you should expose the failure messages
> via the `failure_message` and `failure_message_when_negated` methods.

Since `negative_failure_message` is being added back in with alias_method this will continue to work for users of RSpec 2 (e.g. webmock's own test suite).

What I did loosely matches [the fix that was applied to capybara](https://github.com/jnicklas/capybara/commit/2121ff12fb58ed2f8e4fff095a7ac9555da02f9e).