Update matchers for RSpec 3's matcher protocol

Resolves this deprecation message from 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.
Merge pull request #372 from robolson/rspec3-matcher-compat

Update matchers for RSpec 3's matcher protocol