[Crossbow] Send notifications only for failures

[asfimport]

In order to reduce the notification noise:

• Travis: https://docs.travis-ci.com/user/notifications/#changing-notification-frequency
• Appveyor: https://www.appveyor.com/docs/notifications/#triggering-notifications

Reporter: Krisztian Szucs / @kszucs

_{Note: This issue was originally created as ARROW-5263. Please see the migration documentation for further details.}

[thisisnic]

This issue hasn't had activity in a long time. If it's still being worked on, please leave a comment. Otherwise, it will be closed on 23rd June.

Labelled Status: Stale-Warning for tracking.