force phantomjs service to terminate (see https://github.com/SeleniumHQ/selenium/issues/767)
force phantomjs service to terminate (see https://github.com/SeleniumHQ/selenium/issues/767)

jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader