Build/Test Tools: Improve Travis config to pass config validation.

* Add missing `os` and `dist` with the current default values.
* Replace the alias `matrix` with `jobs`.
* Remove unsupported `on_start` for `notifications.slack`.

Fixes #50296.
Built from https://develop.svn.wordpress.org/trunk@47877


git-svn-id: https://core.svn.wordpress.org/trunk@47651 1a063a9b-81f0-0310-95a4-ce76da25c4cd