Remove Travis-CI integration

Travis-CI has changed their pricing structure for open source projects
which effectively potentially severely limits the number of CI runs
such projects are allowed. We have already a working GitHub workflow
and can safely disable Travis-CI integration.

[1] https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

Signed-off-by: Nicolas Bock <[email protected]>

Author: nicolasbock