Run Github Action tests against Ruby 2.5, 2.6, 2.7

[bensheldon]

GitHub Actions has a matrix feature and I was imagining the test action could be parameterized to test against both those versions. https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-running-with-more-than-one-version-of-nodejs

There is currently a .ruby-version file (I find this very useful for local gem development), so that would likely need to be overwritten/removed before Ruby is installed in the action.