rebaseline_server: use just skpdiff, not Python Image Library

gm/rebaseline_server/imagepair_test.py

 #!/usr/bin/python
 
 """
 Copyright 2014 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 Test imagepair.py
 """
 
 # System-level imports
 import shutil
 import tempfile
 import unittest
 
 # Local imports
 import imagediffdb
 import imagepair
 
 
-IMG_URL_BASE = 'http://chromium-skia-gm.commondatastorage.googleapis.com/gm/bitmap-64bitMD5/'
+IMG_URL_BASE = ('http://chromium-skia-gm.commondatastorage.googleapis.com/'
+                'gm/bitmap-64bitMD5/')
 
 
 class ImagePairTest(unittest.TestCase):
 
   def setUp(self):
     self._temp_dir = tempfile.mkdtemp()
     self.maxDiff = None
 
   def tearDown(self):
     shutil.rmtree(self._temp_dir)
@@ skipping 47 matching lines @@
             'arcofzorro/16206093933823793653.png',
             'arcofzorro/13786535001616823825.png',
             None,
             None,
             # expected output:
             {
                 'differenceData': {
                     'maxDiffPerChannel': [255, 255, 247],
                     'numDifferingPixels': 662,
                     'percentDifferingPixels': 0.0662,
-                    'perceptualDifference': 0.06620000000000914,
+                    'perceptualDifference': 0.06620300000000157,
                 },
                 'imageAUrl': 'arcofzorro/16206093933823793653.png',
                 'imageBUrl': 'arcofzorro/13786535001616823825.png',
                 'isDifferent': True,
             },
         ],
 
         [
             # inputs:
             'gradients_degenerate_2pt/10552995703607727960.png',
@@ skipping 73 matching lines @@
       self.assertEqual(image_pair.as_dict(), selftest[4])
 
 
 def main():
   suite = unittest.TestLoader().loadTestsFromTestCase(ImagePairTest)
   unittest.TextTestRunner(verbosity=2).run(suite)
 
 
 if __name__ == '__main__':
   main()