Add CI validation for the README snippets about sbt-extras

[mcanlas]

Are there projects that already have a mechanism for this?

[dwijnand]

Not really. https://github.com/tpolecat/tut is the closest thing.

We can go the high-tech route of parsing the markdown and extracting code blocks of a given shape, say those that start with "```bash", and then validate those.

Or we can go the low-tech route and use markers (invisible in the rendered form) to chop out the code block and validate it.

Here I'd go with the latter, YAGNI route.

[dwijnand]

Perhaps CI could push "Update README shell session snippets" commits rather than fail the build..

[dwijnand]

I think there's some prior art one could look in @alexarchambault's handy work in https://github.com/coursier/coursier.

[alexarchambault]

@dwijnand I removed the need to update the README some time ago… It was handled all by sbt-release, and all was running on my machine (not the CI).

[dwijnand]

This would require having a java executable in CI, either a real one or a mocked up one, and I think I'd rather leave this using the minimal TravisCI image & not doing the mocking.

So I guess we can keep using adult supervision.