Fix SourceAnnotationExtractor on Rails 6

[brian-kephart]

Without this fix, my tests fail when loading RSpec.
uninitialized constant RSpec::Rails::SourceAnnotationExtractor

With the fix, the constant is appropriately nested under the top-level Rails constant instead of trying to nest under RSpec, so my tests pass again.

[brian-kephart]

See build errors here: https://travis-ci.com/brian-kephart/camaleon-cms/jobs/125697021

And the last comment here: #1984

[brian-kephart]

When I submitted this PR, the rails-controller-testing gem would not load in Rails 6, so the effect of this PR on the test suite could not be evaluated directly. This changed allowed a repo that depends on rspec-rails to build correctly, which was indirect evidence of its effect.

Since then, rails-controller-testing has been updated, so now the tests can be run on Rails master (at least until another problem is encountered). See results here.

With the updated rails-controller-testing gem but without the change from this PR, this is the result.

[nbulaj]

I'm using master branch of RSpec with Rails master and still see this error also:

https://travis-ci.org/doorkeeper-gem/doorkeeper/jobs/380759360

[JonRowe]

Thanks for this!

[dsandstrom]

If you encounter a similar error after updating to Rails 6.1, try updating rspec to 3.10+ and rspec-rails to 4.0+.