Update travis config
Update travis config

jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,MatKallada/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,MatKallada/nbgrader,modulexcite/nbgrader,dementrock/nbgrader,jupyter/nbgrader,jupyter/nbgrader,modulexcite/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,dementrock/nbgrader