Fix for issue-1001: added code to update group coverage when graders are randomly assigned to criteria

#1001

In graders_controller.rb, there is no update action in the randomly_assign_graders_to_criteria method to update group coverage (such action exists in add_graders_to_criteria method). Therefore the group coverage remains unchanged when randomly assigning graders. The required update code is added to randomly_assign_graders_to_criteria method.