Add 'prepend' support to RSpec configuration

RE: #1789 
There was some remaining ambiguity as to what the best approach was for this feature on non-compatible versions. I went the route of checking for a `prepend` definition on `Module` and otherwise raising a `NotImplementedError`; if explicitly checking the `RUBY_VERSION` is the preferred route let me know. For the specs, I conditionally executed them based on the same criteria.
Ok. I'm pretty sure that if I wrap this in a conditional then the need for the other conditonal goes away completely. Is it a valid use-case to directly modify the `include_extend_or_prepend_modules` array? If so, or if we just want to be extremely defensive, then I can leave both checks.