[Security] Bump django from 1.11.20 to 1.11.23

Bumps [django](https://github.com/django/django) from 1.11.20 to 1.11.23. **This update includes security fixes.**
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.20...1.11.23)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -1,7 +1,7 @@
 certifi==2019.3.9
 chardet==3.0.4
 dj-database-url==0.5.0
-Django==1.11.20
+Django==1.11.23
 django-dotenv==1.4.2
 django-webpack-loader==0.6.0
 eventbrite==3.3.3