Add support for Travis

[fproulx-boostsecurity]

Looks like Travis can have pwn request etc.
https://docs.travis-ci.com/user/pull-requests#share-encrypted-environment-variables-with-forks-prs

Just that it was a big problem before March 2022, but they have retroactively hardened existing repos, new repos get new safe setting by default

Dangerous settings (similar to CircleCI or Azure DevOps)
Share encrypted environment variables with forks (PRs)
Share SSH keys with forks (PRs)

https://github.com/search?q=language%3Ayaml%20path%3A%2F%5E%5C.travis.ya%3Fml%24%2F%20NOT%20is%3Afork&type=code