Cleanup .travis.yml.
Cleanup .travis.yml.

* Add `docs`.
* Only run coveralls on `main`.
* Drop unneeded `secure` var.

jonparrott/oauth2client,clancychilds/oauth2client,google/oauth2client,clancychilds/oauth2client,google/oauth2client,googleapis/oauth2client,jonparrott/oauth2client,googleapis/oauth2client