Updated matchers to work with rspec 3.0.0

When writing tests for abilities, using rspec 3.0.0 I got this message:

failure_message_for_should_not is deprecated. Use failure_message_when_negated instead. Called from /home/vagrant/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/cancancan-1.7.0/lib/cancan/matchers.rb:11

failure_message_for_should is deprecated. Use failure_message instead. Called from /home/vagrant/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/cancancan-1.7.0/lib/cancan/matchers.rb:7