[TASK] Change phpunit repository url for travis

Resolves: #55366
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: Ia90d7c85f81828bd23275b7fa4017ee74a758ad6
Reviewed-on: https://review.typo3.org/27074
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny