Switch to travis-ci.com as default

[riccardoporreca]

With the support for open source projects on travis-ci.com, it is actually recommended to actually use travis-ci.com, especially for new repositories (https://docs.travis-ci.com/user/migrate/open-source-on-travis-ci-com/)

The suggestion here is to align the argument ext to use_travis() and friends to use "com" as default. Happy to contribute a PR.

[hadley]

Given that we're moving away from travis in favour of github actions, I think the only way we'd do this is through a PR 😄

[riccardoporreca]

@hadley: sure, the efforts towards github actions are highly appreciated ;)

PR coming soon...