[TASK] Reformat .travis.yml to be better readable

Resolves: #51119
Releases: 6.2
Change-Id: Ibbabf4de13aa4bff069021629bedb14fd111f091
Reviewed-on: https://review.typo3.org/23136
Reviewed-by: Stefan Neufeind
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn