Downgrade Pillow pip requirement

2.7.0 isn't really necessary - 2.3.0 is sufficient. This is the version
available on Ubuntu 14.04 via apt-get, and seems to be a reasonable lowest
common denominator in general.

http://pillow.readthedocs.org/installation.html#old-versions

python/requirements.txt

@@ -14,4 +14,4 @@ python-dateutil>=1.4,<2
 protobuf>=2.5.0
 python-gflags>=2.0
 pyyaml>=3.10
-Pillow>=2.7.0
+Pillow>=2.3.0