Update dependency python-dateutil to v2.7.4

Author: renovate-bot

requirements.txt

@@ -12,7 +12,7 @@ itsdangerous==0.24
 Jinja2==2.9.6
 Mako==1.0.7
 MarkupSafe==1.0
-python-dateutil==2.6.1
+python-dateutil==2.7.4
 python-editor==1.0.3
 requests==2.18.4
 six==1.11.0