rspec 3 expects pending steps to fail

In RSpec 3, when a spec is pending and passes, it returns an error like "Expected pending 'No reason given' to fail. No Error was raised", and is treated as a failure.  When a spec is pending and fails, it behaves like a pending spec did in RSpec 2.  This pull request makes simple_bdd behave as it did in RSpec 2 by raising an error either way, and only marking the step as pending if RSpec.configuration.raise_error_on_missing_step_implementation is set to false.