Migrating CI to travis-ci.com

[Eder-K]

See https://docs.travis-ci.com/user/migrate/open-source-repository-migration

With the sunsetting of travis-ci.org at 31.12, it is necessary to transfer the CI-related features of this repository to the new .com-domain. This likely includes a number of changes both on the TravisCI webpage as well as in the codebase.

Note: this transition is a necessary step for continued CI support in the short-term. We still intend to switch off of TravisCI (relevant issue: #253) eventually.