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
 """
@@ skipping 69 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,

[epoger, 2014/06/12 07:02:07]

There are minute changes to the perceptual differences we see after this CL.  I
guess that's because we now read that value out of skpdiff's JSON output file,
not its CSV output file.

                 },
                 '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()