Add automated testing via GitHub actions workflow

[joeyparis]

Objective

Automatically run rspec tests on multiple ruby versions on pull requests and code pushes.

Notes

I did update the bundler dev dependency to 2.2, if this is undesirable I can downgrade the bundler that GitHub uses by default.

How to test

You can run these tests locally using https://github.com/nektos/act and running act -j test. You'll want to add ImageOS=ubuntu18 to your .env file and create a file named .actrc with the contents -P ubuntu-latest=nektos/act-environments-ubuntu:18.04. (Note running this the first time will require a large download, you can substitute the ubuntu-latest value with catthehacker/ubuntu:act-18.04 for a medium-size image and node:12.20.1-buster-slim for a small image, although you'd need to add additional dependencies to the workflow

Breaking changes

N/A

Checklist

• I have read the CONTRIBUTING document.
• I have created a Codelog changefile with the changes made to the branch.
• I have created a test proving that my feature/fix does what it intends to do.
• I have updated the documentation accordingly. (If needed)