[TASK] travis-ci: Execute unit test only

travis-ci still chockes on executing the functional tests
which take ages, even with various tricks in place.
The patch reduces the travis execution to unit tests
only since all main tests are executed via our bamboo
environment anyway beforehand.

Change-Id: I809f38d79f0ac7848e6b7e155d35aab575f4d81a
Resolves: #82172
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/53780


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>