Fix matcher compatibility with RSpec 2

I messed up with #156. I was thinking of the other RSpec matcher syntax where you define `failure_message_when_negated` as a method, rather than use it as a DSL call...

This way of retaining compatibility with both RSpec 2 & 3 is taken straight from the [corresponding file in the `pundit` gem](https://github.com/elabs/pundit/blob/v0.3.0/lib/pundit/rspec.rb).