Fix AssertionError in TestFileJpeg.test_save_cjpeg

Fixes the following test failure on Windows:
```
======================================================================
FAIL: TestFileJpeg.test_save_cjpeg
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\Build\Pillow\Pillow-git\Tests\test_file_jpeg.py", line 326, in test_save_cjpeg
    self.assert_image_similar(img, Image.open(tempfile), 15)
  File "D:\Build\Pillow\Pillow-git\Tests\helper.py", line 105, in assert_image_similar
    ave_diff, epsilon))
AssertionError: 15.0 not greater than or equal to 16.4700927734375 :  average pixel value difference 16.4701 > epsilon 15.0000
```