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
Version bump to 1.17.4
Merge branch 'master' of github.com:bblimke/webmock into addressable

Conflicts:
	README.md