Migrate to travis-ci.com

[ranocha]

I noticed that this repo still uses travic-ci.org, at least based on the badge in the README. travis-ci.org will be shut down at the end of the year, cf. https://mailchi.mp/3d439eeb1098/travis-ciorg-is-moving-to-travis-cicom. Hence, everything should be migrated to travis-ci.com.

[piever]

Thanks for reporting, closing this as we moved to github actions.