travis_enable: Activate Travis CI
In romainfrancois/travis: Set Up 'Travis' for Testing and Deployment

Activating Travis CI, and querying activity status.

travis_enable() activates or deactivates Travis CI for a repo.

travis_is_enabled() returns if Travis CI is active for a repo.

travis_enable(active = TRUE, repo = github_repo(),
  token = travis_token(repo = repo), quiet = FALSE)

travis_is_enabled(repo = github_repo(), token = travis_token(repo))

`active`	`[flag]` Set to `FALSE` to deactivate instead of activating.
`repo`	`[string\|numeric]` The GitHub repo slug, by default obtained through `github_repo()`. Alternatively, the Travis CI repo ID, e.g. obtained through `travis_repo_id()`.
`token`	`[Token2.0]` A Travis CI token obtained from `travis_token()` or `auth_travis()`.
`quiet`	`[flag]` Set to `FALSE` to suppress success message.