Bump requests from 2.9.1 to 2.18.4

Bumps [requests](https://github.com/requests/requests) from 2.9.1 to 2.18.4.
- [Changelog](https://github.com/requests/requests/blob/master/HISTORY.rst)
- [Commits](https://github.com/requests/requests/compare/v2.9.1...v2.18.4)

requirements.txt

@@ -13,7 +13,7 @@ prettytable==0.7.2
 # Don't forget to update the version of this installed in
 # the aws role as well.
 awscli==1.11.58
-requests==2.9.1
+requests==2.18.4
 datadog==0.8.0
 networkx==1.11
 pathlib2==2.1.0