Tests for Ruby 2.7 and 3.0

[Hiroaki Osawa]

Hi! Thank you for great gem.
Could you please add Ruby 3.0 and 2.7 tests as shown in the ranked-model commit below?
brendon/ranked-model#174
brendon/ranked-model#168

I didn't think it would be a good idea for me to just copy and paste this code and I will be a contributor, since it's mostly the result of @aried3r efforts!

[Anton Rieder]

Hey @QWYNG, while I don't use this gem at my current work project, I'm perfectly fine if you copy and paste what I did for ranked-model. :)

Cheers

[Hiroaki Osawa]

@aried3r
hmm, Thanks for your kind offer. I appreciate it.
created PR #393

[Brendon Muir]

All good @QWYNG, I'll look to merge your PR.