Fix rspec 2.14 deprecation warnings

This is a preemptive fix for deprecation warnings in rspec 2.14. It turned out that the only deprecation that affects should matchers is "expect { }.not_to raise_error(SpecificErrorClass)" (see rspec/rspec-expectations#231).

shoulda-matchers is not currently using rspec 2.14 but I upgraded on a branch of my own, which is how I came across these warnings.