Enable Travis Auto Cancellation

[aneeshusa]

This is a new beta feature: https://blog.travis-ci.com/2017-03-22-introducing-auto-cancellation

When iterating on a branch and force pushing frequently, this will automatically cancel old builds that haven't yet started to avoid running builds which would have stale results.

cc @edunham @larsbergstrom @metajack

[larsbergstrom]

That sounds great! Do you think we should enable it on all of our repos? It looks like it's a per-project setting.

[metajack]

I think we should enable the PR auto-cancel but not the branch auto-cancel. That way we'll still know which specific commit introduced a failure. I've enabled PR auto-cancel for most of the repos in the servo org. I wish there was a way to control this more easily than clicking into every repo on Travis CI and then hitting settings and flipping the toggle.

[aneeshusa]

@metajack In their beta issue tracker, the Travis CI folks mention this can be modified via the API. I think it might be nice to make a script for this (similar to the ones in #457).

[aneeshusa]

In the meantime, it might be nice to add this to the checklist for adding new repos on the wiki.

[metajack]

Made the wiki changes.

[aneeshusa]

This is enabled and documented, so I don't see any other action items for this right now. Closing.