Satisfy Rubocop's guard clause expectations.

Big thanks (づ｡◕‿‿◕｡)づ to @mikegee for the suggestion in
https://github.com/rspec/rspec-rails/commit/f6c277f8cb#commitcomment-7167216.
That provided the vast majority of the refactor.

At the end of the day, this changes makes a lot more sense, separating
concerns in a more logical location.
Note where to add requires so Rails is loaded.

There are no clear directions where additional library requires should
be included. This let's users know where to add the requires so that
Rails is loaded and availble for them.
Backport pull request #1136 and #1139 into 3-0-maintenance