updates thumbor to 6.4.2

resolves a thumbor issue with png files
thumbor/thumbor#943

Author: decafdev

source/image-handler/setup.py

@@ -26,7 +26,7 @@
         'botocore==1.3.7',
         'tornado_botocore==1.0.2',
         'requests_unixsocket>=0.1.5',
-        'thumbor>=6.2.1',
+        'thumbor>=6.4.2',
         'tc_aws==6.0.3',
         'opencv-python==3.2.0.6'
     ],