Use RSpec.current_scope

[pirj]

It was introduced in rspec/rspec-core#2895

When RSpec.current_scope is released in 3.11 and 4.0, we're fine with merging this PR to main and to 5-0-maintenance.

TODO:

• remove the temp! commit

[JonRowe]

For when the other PR is merged, this gets a thumbs up (without the temp pin commit of course).

[pirj]

If I merge now, I'll surely forget to cherry-pick to 5-0-maintenance.

If I cherry-pick it to 5-0-maintenance, I would have to bump expected_rspec_version in gemspec to the yet unreleased 3.11.0.

In theory, pointing to 3.11.0 might break someone's setup if they prefer using 5-0-maintenance with the latest unreleased fixes to 5.0.x.
In practice, though, there are no changes since 5.0.2, so for those using github: ..., branch ... there's a workaround to use ~> 5.0.

If I forget to cherry-pick to 5-0-maintenance, it will only be available for the future 6.0 release. Not a big deal, though.

This process requires thinking and having good memory.
I intend to hold off merging this until 3.11.0 is released, does this sound reasonable?

[JonRowe]

I don't think this should be merged to 5-0-maintenance as I don't think its a good idea to bump a dependency in a patch version, only a minor. However this also would gate any release of rspec-rails5.1 on 3.11 so its probably still worth holding off merging.