Add travis.yml for gh-pages
Add travis.yml for gh-pages

I think this is needed on this branch to get rid of the message that a
build config is needed for the gh-pages branch.

See
https://docs.travis-ci.com/user/customizing-the-build/#Building-Specific-Branches

Especially the note that travis.yml needs to be present on all active
branches.

bsittler/async-cookies-api,bsittler/async-cookies-api