> Having just learned of Travis on this thread, I can't seem to find its source code.

https://github.com/travis-ci/travis-ci

I don't think there's a quick and easy way to set up your local version of Travis, but it does seem possible.

> But is Travis really a tool or just a service?

I would certainly argue it's more on the 'service' end of things. If I had to guess, most people who find hosting their code on Github acceptable (Travis only works with Github at the moment) also find using the Travis web service acceptable as well so there isn't a lot of incentive to use Travis as a locally installed tool rather than using their as-a-service offering.