Fixes for RSpec 3.13.0

[jas14]

Restored from #251:

Added an Appraisal and implemented a couple fixes for compatibility with RSpec 3.13.0+. 🙂 This closes #250.

I'll admit that the version conditionals directly in the monkey patch file is rough. That file itself is also getting pretty large. I considered splitting up the patches into a monkey_patches/ directory and having each file define the version(s) to which it applies. Let me know if that sounds good and I'll be happy to implement it, either here or in a follow-up PR.