migrate away from Travis CI

[dneuge]

Unnoticed, Travis CI stopped offering non-bureaucratic free build service. Paid accounts make no sense as free or cheaper alternatives exist. Instead of applying for an OSS account the project should be migrated away from Travis CI, alternative services should be re-evaluated.

• GitHub Actions was originally not chosen as it required an unverifiable 3rd-party script for GitHub release creation which breaks trust in binary artifacts.
• AppVeyor was just not chosen because I was already using Travis CI for other projects at that time which works almost the same.
• Private infrastructure was not chosen due to time constraints (Jenkins) and open security/server management questions regarding Gitlab Runner (also mainly time constraints) and difficult/missing GitHub integration to free self-hosted Gitlab instances.

CI builds for dependencies (web-data-retrieval and VATPlanner) should be covered by the same build service if possible.

[dneuge]

Own infrastructure might be necessary to integrate a closed-source update check (see #14).

[dneuge]

Travis has blocked further use of the remaining, originally granted credits. This may be a bug on their end but it's a good opportunity to migrate away now.

Releases are blocked until resolved.

[dneuge]

Migrated to private Gitlab instance.