Update cb2.gemspec to support redis 4.x and 5.x gem versions

[acunningham-fz]

PR to support issue: #16

[prognostikos]

@acunningham-fz I think you'll need to run bundle update --conservative and check in the changes to Gemfile.lock in order for the tests to run

[acunningham-fz]

ugh - apologies @prognostikos - I should know better than to put up a PR without doing the local verification. The smallest changes are always the ones that bite me 😆 .

Done the update of the Gemfile.lock but have maybe gone too far now as have updated the version of ruby to test against to reflect current 'in-support' versions. Happy to re-add 3.0, 3.1 if needed. Thanks.

[prognostikos]

@acunningham-fz thanks it looks good. One thing that could be improved is changing CI so that we explicitly test against the latest 4.x and 5.x versions of the redis gem. However if you're not interested in that let me know and I will merge as-is.

[prognostikos]

I'll merge this and add the tests.

[acunningham-fz]

thank you! sorry, I actually found a nice pattern using appraisal but hadn't found the time to give it a go but saw your solution which is pretty much the same so will leave it. Thanks for taking the time to work through this, very much appreciate it.