Noop ruby probes

On a platform where DTrace cannot be found, an empty C extension is used instead of the one that hooks into libusdt. In ruby land, a stub implementation is then required.

I also added some specs around the ruby stubs. I tried to write minitests instead, but was fighting with the framework for long enough that I gave up in favor of rspec. If you find rspec objectionable it should be replaceable.