Add GitHub Actions Workflow for Test Suite

[nskins]

I added a workflow to automatically run the test suite on PRs and pushes.

[nskins]

I had to set this up with the GitHub Actions wizard because I couldn't get the workflow to trigger otherwise. The source branch was branched off master (I didn't have a choice) and, as a result, the push workflow fails because it spots the old version of bundler in goby.gemspec. However, the PR workflow succeeds (which is what we care about) because the destination branch has the updated version of bundler specified in goby.gemspec. Should be good to go here.