Fix call to subprocess
Fix call to subprocess

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