Add 'prepend' support to RSpec configuration

Created by: arlandism

RE: #1789 (closed) 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.