Update six to 1.10.0.
Update six to 1.10.0.

A new setuptools was released which depends on this and it is breaking Travis CI that we are installing an old one.

https://travis-ci.org/galaxyproject/galaxy/jobs/194699159.

galaxyproject/starforge,galaxyproject/starforge,natefoo/starforge,natefoo/starforge,bgruening/docker-build,bgruening/docker-build