Run the test suite on GitHub Actions

[facundofarias]

Problem

The repository has no CI configuration of any kind, so the RSpec suite only ever runs locally. A change that breaks it can be merged without anyone noticing, and contributors get no signal on a pull request.

Change

A single workflow that runs rspec on pushes to master and on every pull request, across Ruby 2.7 through 3.4.

Notes on the approach

The suite is run without Bundler. Procodile has no runtime dependencies outside the standard library, and the committed Gemfile.lock pins rspec 3.5.x (2016) and json 2.2.0, which do not install on current Rubies. Installing rspec directly keeps the whole matrix green without changing or deleting the lockfile — that felt out of scope for a PR whose job is just to switch CI on. Happy to swap to bundler-cache: true instead if you'd rather drop the committed lockfile (which is the more usual convention for a gem); that's a one-line change here plus removing the file.

Ruby 2.7 runs on ubuntu-22.04 because it isn't available on the ubuntu-24.04 runner image that ubuntu-latest now resolves to. Everything else runs on ubuntu-latest.

Verification

Run locally against the Rubies I had available — 2.7.8, 3.2.2, 3.3.8 and 3.4.9 — 34 examples, 0 failures on each. 3.0 and 3.1 are in the matrix but I couldn't test them locally; they sit between two verified versions and the gem is pure Ruby, so I'd expect them to pass, and CI will say so definitively on this PR.

fail-fast: false so one failing Ruby doesn't hide the others.

If you'd like a build badge in the README I'm happy to add one, but I've left it out to keep this focused.

[facundofarias]

Update on the two Rubies I couldn't test locally: the matrix has now been run end-to-end and all six jobs pass.

Ruby	Runner	Result
2.7	ubuntu-22.04	pass (46s)
3.0	ubuntu-latest	pass (5s)
3.1	ubuntu-latest	pass (8s)
3.2	ubuntu-latest	pass (10s)
3.3	ubuntu-latest	pass (8s)
3.4	ubuntu-latest	pass (6s)

So 3.0 and 3.1 are confirmed rather than assumed, and the ubuntu-22.04 pin for 2.7 is confirmed necessary — that job builds Ruby 2.7 from source (hence 46s vs 5-10s elsewhere) because no prebuilt 2.7 exists for the 24.04 image.

Run: https://github.com/deployhq/procodile/actions/runs/34689316464

I ran it on a fork-side copy of this PR because GitHub needs a maintainer to approve workflows on a first-time contributor's PR, so no checks appear here yet. Approving them on this PR should reproduce exactly the same result.