[TASK] Use 6.1 branch in travis-integration for travis

Change-Id: I748bae0a4c4995d6e47b42d2d84cead36db3665d
Resolves: #52731
Related: #47018
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/24660
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn