Fix rspec invocation in CircleCI

[jordan-brough]

The tests weren't actually running for Rails >= v6. Only the "bundler:audit" task was.

E.g. see the output and the test time (only 3s) in this CircleCI run:

See also the bundle exec appraisal rake part of that same test run. The tests only ran for Rails < v6.

[jordan-brough]

@pablobm @nickcharlton (I'm not 100% who the maintainers are currently) - I know I've opened a lot of PRs recently, but I think this one is probably the most important one. If you have a minute to take a look sometime that would be great. Thanks! 🙏

[pablobm]

@jordan-brough - Thank you so much for all the PRs. Tiny and focused; the best sort of PR! The only reason I'm not merging everything just yet is because I'm having an issue running my specs locally, and it makes it difficult to check things. As soon as I solve this, I'll go through all of them.

I'm going to make an exception with this one and merge it now because as you say it's pretty important (and also embarrassing!).

[jordan-brough]

Thanks @pablobm! FYI I'm going to go through and rebase all my PRs on main so that they retrigger tests w/ this update in place.

[jordan-brough]

@pablobm FYI I've rebased them all and they're all green.

Though a couple (2127 and 2128) had some test-flakiness that I don't think is related to the changes.