Bump urllib3 from 1.24.2 to 1.25.3 in /app

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.24.2 to 1.25.3.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/master/CHANGES.rst)
- [Commits](https://github.com/urllib3/urllib3/compare/1.24.2...1.25.3)

app/requirements.txt

@@ -29,7 +29,7 @@ pyparsing==2.3.0
 python-memcached==1.59
 requests==2.21.0
 six==1.12.0
-urllib3==1.24.2
+urllib3==1.25.3
 uWSGI==2.0.17.1
 visitor==0.1.3
 Werkzeug==0.14.1