supported rspec3 matcher protocols

The following message is displayed when testing with rspec-rails v3.0.0beta.

```
Deprecation Warnings:

--------------------------------------------------------------------------
CarrierWave::Test::Matchers::BeIdenticalTo 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.
(Used from /home/vagrant/carrier/spec/uploaders/my_uploader_spec.rb:33:in `block (2 levels) in <top (required)>')
--------------------------------------------------------------------------
```

So I added `failure_message_when_negated` method.
(And `negative_failure_message` method is left for lower compatibility.)

Documentation is here:
https://www.relishapp.com/rspec/rspec-expectations/v/3-0/docs/custom-matchers/define-matcher#overriding-the-failure-message-when-negated