travis: Don't use --use-mirrors. [skip ci]
travis: Don't use --use-mirrors. [skip ci]

PyPi is moving away from the DNS mirrors and instead is serving content
through a CDN now.

amwilson/pypuppetdb,jcastillocano/pypuppetdb,jorik041/pypuppetdb,dforste/pypuppetdb,voxpupuli/pypuppetdb,vicinus/pypuppetdb,puppet-community/pypuppetdb,jcastillocano/pypuppetdb