Remove reference to loading rails in spec helper documentation.

We haven't updated rspec-rails to not do this yet, and we should be
consistent. Added an extra sentence suggesting the direction we are
moving.

Related: writing that sentence without using "require" three times was a
difficult exercise.

We can revert this (or at least add back the rails suggestion) once https://github.com/rspec/rspec-rails/issues/990 is addressed.

@myronmarston @alindeman