Update README gif links after docs move into nbgrader
Update README gif links after docs move into nbgrader

jhamrick/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader