RSpec 3 compatibility: Provide failure_message_when_negated for rspec matcher

This fixes RSpec 3's deprecation warnings:

```
Deprecation Warnings:

--------------------------------------------------------------------------------
ActiveAttr::Matchers::HaveAttributeMatcher 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 ../spec/models/post_spec.rb:14:in `block (3 levels) in <top (required)>')
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
ActiveAttr::Matchers::HaveAttributeMatcher 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 ../spec/models/post_spec.rb:11:in `block (3 levels) in <top (required)>')
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
ActiveAttr::Matchers::HaveAttributeMatcher 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 ../spec/models/post_spec.rb:15:in `block (3 levels) in <top (required)>')
```