Add CI check for ocamlformat

[naartjie]

Ocamlformat has been added to the project via #177.

I'm opening up this issue to track the fact that we still need to add a CI check for it.

I had an initial look at it, but to be honest I don't really know how TravisCI works, nor the ocaml build pipeline in travis. If anyone has any suggestions or tips, please comment here, thanks.

[andreas]

If it makes a difference, I'd be perfectly happy with a different CI setup also (Travis is very slow!), e.g. based on Github Actions, CircleCI or similar.

[wokalski]

here's a validation workflow from brisk-reconciler

https://github.com/briskml/brisk-reconciler/blob/d185e75c47b381fb911ca9decdf7525b97188ea1/.github/workflows/validate-pr.yml

[naartjie]

Great idea. GitHub actions sounds perfect for this.

[wokalski]

It even posts a comment when linter has spotted an issue. Amazing!