In Integration with TravisCI chapter I see the example of .travis.yml that is using pip install html5validator command, but it fails with "permission denied" error. In this case we should use --user option: http://stackoverflow.com/questions/27036259/how-to-write-travis-yml-if-my-project-deps-on-python-and-nodejs

Probably, it also affects old-style builds, that don't use the docker infrastructure.