update .travis.yml to point to gini/puppet-archive

I was looking at the `.travis.yml` to figure out how to build the project locally when I noticed it's cloning [smarchive/puppet-archive][original]. The only file in that repository currently is a README that says the project has moved to [gini/puppet-archive][current].

This update reflects the change in that README.

[original]: https://github.com/smarchive/puppet-archive
[current]: https://github.com/gini/puppet-archive