Re-add CI builds

[maelle]

• We no longer recommend using Travis and actually travis-ci.org is down https://ropensci.org/blog/2020/11/19/moving-away-travis/
• You could use GitHub Actions https://devdevguide.netlify.app/ci.html e.g. via usethis::use_github_action_check_standard()

It is useful in general and would help with our discussion in #8

[maelle]

Thanks @jdeck88