update to django 2.0

submission/views/oj.py

@@ -198,6 +198,6 @@ class SubmissionExistsAPI(APIView):
     def get(self, request):
         if not request.GET.get("problem_id"):
             return self.error("Parameter error, problem_id is required")
-        return self.success(request.user.is_authenticated() and
+        return self.success(request.user.is_authenticated and
                             Submission.objects.filter(problem_id=request.GET["problem_id"],
                                                       user_id=request.user.id).exists())