Bump jinja2 from 2.10.1 to 2.11.3 in /api

Bumps [jinja2](https://github.com/pallets/jinja) from 2.10.1 to 2.11.3.
- [Release notes](https://github.com/pallets/jinja/releases)
- [Changelog](https://github.com/pallets/jinja/blob/master/CHANGES.rst)
- [Commits](pallets/jinja@2.10.1...2.11.3)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

api/requirements.txt

@@ -5,7 +5,7 @@ celery==4.2.1
 click==6.7
 Flask==1.0.2
 itsdangerous==0.24
-Jinja2==2.10.1
+Jinja2==2.11.3
 kombu==4.2.0
 redis==2.10.6
 tornado==5.0.2