Added Pillow to dev requirements

requirements-dev.txt

@@ -1,6 +1,9 @@
-# For coverage and PEP8 linting
-coverage>=3.7.0
-flake8>=2.2.0
+# Required for running the tests
 mock>=1.0.0
 python-dateutil>=2.2
 pytz>=2014.7
+Pillow>=2.7.0
+
+# For coverage and PEP8 linting
+coverage>=3.7.0
+flake8>=2.2.0