add minimal-versions to CI

[Eh2406]

This starts work on the request to add minimal-versions to CI.

This tests all configuration with minimal-versions which is a very aggressive testing resheam. But is a place to start the conversation, and seemed to be easy to do with the way travis is set up.

[BurntSushi]

What do you think about just running these commands when the nightly matrix entry runs, instead of on every build?

[Eh2406]

That is what I meant with my question about configurations. Is there some easy way to do an if nightly in this style of travis file?

[BurntSushi]

Oh I see what you meant now. Sorry, didn't catch that. And yeah:

script:
  - cargo build --verbose
  - cargo test --verbose
  - cargo doc
  - if [ "$TRAVIS_RUST_VERSION" = "nightly" ]; then
      cargo +nightly generate-lockfile -Z minimal-versions;
      cargo build --verbose;
      cargo test --verbose;
    fi

(Note the semi-colons. This is usually when I start splitting the script section to a shell script, but I'm fine with this for now. I can fix that later if I care.)

[BurntSushi]

Thanks!

[Eh2406]

I believe this is the second crate to have this in CI, So if you have feedback on the experience please report it at the tracking issue.

[BurntSushi]

@Eh2406 Great! If you submit more PRs to my other crates, then I'd recommend just bundling the CI config with that, including regex. As it is, docopt doesn't get much action, so I doubt this specific change will generate much feedback. :-)

[BurntSushi]

(But if you managed to get all the way to ripgrep, for example---which I'd be willing to try---then that will definitely get you feedback.)