RSpec `failure_message_for_should_not` is deprecated.

I am getting this deprecation warning when using Rails 4.0.3, Rspec 3.0.0.beta2 and cancan 1.6.10.

```
`failure_message_for_should_not` is deprecated. Use `failure_message_when_negated`
instead. Called from /home/vagrant/.rvm/gems/ruby-2.1.1/gems/cancan-
1.6.10/lib/cancan/matchers.rb:11:in `block in <top (required)>'.
```
