Run fewer Travis jobs during Pull Requests.

[iphydf]

Still run all the jobs during cron and regular push to branch. We
disabled build for push to branch, so cron is the only place where all
builds are run. This also means we need to worry less about spending time
in nightly builds, because they occur only once a day.

See travis-ci/travis-ci#9071

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[e0ff]

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[nurupo]

It's a sad PR but if the long build times are that much of an issue then alright.

Where can one see the results of cron builds? I'm afraid that a build failure in the cron build might go unnoticed.

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Comments from Reviewable

[iphydf]

The cron builds end up in the build history and on IRC. You're right, it has a higher chance of going unnoticed. I'd say we try it and see if we end up having failures go unnoticed. In the meantime, we speed up development by a factor of about 18.

[nurupo]

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Comments from Reviewable

[nurupo]

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Reviewed 1 of 2 files at r3.
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