Update python-dateutil from 2.7.3 to 2.7.5 (#776)

Author: pyup-bot

requirements.txt

@@ -12,7 +12,7 @@ pytest-cover==3.0.0
 pytest-randomly==1.2.3
 pytest-tornado==0.5.0
 pytest==3.9.2
-python-dateutil==2.7.3
+python-dateutil==2.7.5
 pyyaml==3.13
 six==1.11.0
 sphinx-argparse==0.2.3