Fix rspec 2.14 deprecation warnings

This is a preemptive fix for deprecation warnings when running the test suite after upgrading to rspec 2.14. 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. They're only relevant to the test suite.

As it happens, the only deprecation warning that appears is "expect { }.not_to raise_error(SpecificErrorClass)" (see rspec/rspec-expectations#231). Easy fix