Simplify Pillow requirement (issue #470)

Signed-off-by: Michal Čihař <michal@cihar.com>

requirements.txt

@@ -4,5 +4,5 @@ South>=0.7
 Whoosh>=2.5.2
 Translate-Toolkit>=1.10.0
 lxml>=3.1.0
-Pillow>2.4.0,<2.4.0
+Pillow!=2.4.0
 python-social-auth>=0.1.17