Fixed bug on 'raise exc'

At line 572: raise exc() has to be raise exc since exc is an instance not a class

celery/app/task.py

@@ -569,7 +569,7 @@ def retry(self, args=None, kwargs=None, exc=None, throw=True,
                # first try to reraise the original exception
                maybe_reraise()
                # or if not in an except block then raise the custom exc.
                raise exc()
                raise exc
            raise self.MaxRetriesExceededError(
                "Can't retry {0}[{1}] args:{2} kwargs:{3}".format(
                    self.name, request.id, S.args, S.kwargs))