Pillow support for Python 3.6

If you keep it at Pillow 3, you will get this error when running with Python 3.6 on x64 Windows 10 machine. 
``ValueError: zlib is required unless explicitly disabled using --disable-zlib, aborting``

requirements.txt

@@ -10,7 +10,7 @@ fake-factory~=0.7.2
 html5lib<0.99999999,>=0.999
 markdown~=2.6.7
 path.py~=9.0
-pillow~=3.4.2
+pillow~=4.0.0
 psycopg2~=2.6.2
 pytz
 requests<3