implemented edit and update in the project_memberships_controller, and its routes
project_memberships_test
 tests that only admins can edit and update the project
 tests that admins can update all the roles in a project_membership
 tests that admins cannot create a ProjectUser or CoreUser project_membership with the administrator role
 tests that admins cannot update a ProjectUser or CoreUser project_membership to have the administrator role
ALL TESTS PASS