Bump django from 2.2.4 to 2.2.18

Bumps [django](https://github.com/django/django) from 2.2.4 to 2.2.18.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.2.4...2.2.18

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

requirements.txt

@@ -4,7 +4,7 @@
 # NOTE:  The pip Python package must be installed to use this
 # requirements.txt file.
 
-Django==2.2.4
+Django==2.2.18
 django-extensions==2.1.6
 numpy==1.16.2
 numexpr==2.6.9