Update to work with RSpec 3's method of instantiating examples

RSpec 2 provided `example` as an instance method in `ExampleGroup`s, but no more: https://github.com/rspec/rspec-core/blob/3-1-maintenance/lib/rspec/core/example.rb

Happy to hear other ideas to get it to work with RSpec 3.
The `yield` runs the spec step; if it gets past it, then we report that an example has passed so we get extra reporting when they're not actually extra examples.