[TASK] Update Travis CI notification settings

Notify on Slack and IRC, remove email notification.

Resolves: #59838
Releases: 6.3, 6.2, 4.5
Change-Id: Ic4dacd5c7b6b4e6e2b8cfa92ae7976b666209747
Reviewed-on: https://review.typo3.org/31209
Reviewed-by: Nicole Cordes
Reviewed-by: Michael Stucki
Tested-by: Michael Stucki