[BUGFIX] Set master branch alias in travis

Due to the circular dependency with typo3/testing-framework,
composer needs to know the version of the root package.
For that it tries to identify which branch the current checkout is on.

This fails in some situations, so we force the root package version
to be dev-master now.

Change-Id: Ib923f412a74436abd59b480eefb330214498b216
Resolves: #81925
Releases: master
Reviewed-on: https://review.typo3.org/53552


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Helmut Hummel <typo3@helhum.io>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>