Enhancement: Make it possible for users to specify the version of the OpenAI gym lib they want to use #47

I gather from conversation with @AechPro and @lucas-emery that there's a strong contingent of researchers out there who depend on OpenAI gym v0.24.0, and may continue to do so for quite some time.

To be accommodating to those researchers, I think it would be great if distrib-rl had some sort of compatibility layer that supported numerous versions of the gym API spec and ultimately allowed consumers of distrib-rl to bring their prefered version of the gym API into distrib-rl at runtime.

This compatibility layer should document the minimum gym API version supported, and include tests that exercise the bounds of the various API revisions to ensure that we maintain compatibility going forward.