Fix travis install on python 3
Fix travis install on python 3

jhamrick/nbgrader,jupyter/nbgrader,modulexcite/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,jdfreder/nbgrader,jupyter/nbgrader,MatKallada/nbgrader,MatKallada/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,alope107/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,modulexcite/nbgrader,dementrock/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,dementrock/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,alope107/nbgrader,jdfreder/nbgrader,jupyter/nbgrader