Update .gitignore
Update .gitignore

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