BLD: Travis dedup via webhooks

[kawochen]

Came across this just now https://github.com/grosser/travis_dedup
It sounds like it cancels old builds on a PR. It works via github webhooks.

[jreback]

oh that looks nice. ok i'll try this when I have some time.

[jreback]

ok I setup this webhook, let's monitor and see if this works

[gliptak]

@jreback I have seen builds cancelled for out-of-date (repushed) commits when working #12833

[jreback]

that was me :) sometimes I cancel the dups

[jorisvandenbossche]

For inspiration, there is a script in this PR on conda-forge to do something like this: conda-forge/staged-recipes#2257

[jreback]

that's nice will steal!