Fix cucumber issues blocking ruby-head
Created by: petergoldstein
This PR gets CI to green against ruby-head. It does so by:
- Loosening the cucumber version restrictions such that a ruby-head compatible version
- Making the tag syntax logic dynamic, based on the Ruby version
- Removing Fixnum from the described_class spec. (Also added a spec for the outer case)
- Renaming a when condition to avoid a string escape behavior that varies between cucumber versions.
cc: @JonRowe