Correct update in universal perturbation attack

(cherry picked from commit f8d50a6)

Author: Irina Nicolae

art/attacks/newtonfool.py

@@ -1,4 +1,4 @@
-from __future__ import absolute_import, division, print_function
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import numpy as np
 

art/attacks/universal_perturbation.py

@@ -104,7 +104,7 @@ def generate(self, x, **kwargs):
 
                     # If the class has changed, update v
                     if fk_i_hat != adv_fk_i_hat:
-                        v += adv_xi - xi - v
+                        v += adv_xi - xi
 
                         # Project on L_p ball
                         v = self._clip_perturbation(v, self.eps, self.p)