Remove dependency on https://github.com/minrk/travis-wheels, which is now deprecated
Remove dependency on https://github.com/minrk/travis-wheels, which is now deprecated

jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader