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.
Just to clarify, you're saying you prefer the no-op approach, but *if* I were to raise I should move it up to `prepend`?