build(deps): bump django from 2.2.15 to 2.2.18

Bumps [django](https://github.com/django/django) from 2.2.15 to 2.2.18.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.15...2.2.18)

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

requirements.txt

@@ -1,5 +1,5 @@
 XBlock~=1.3.1
 xblock-utils~=2.1.1
-django==2.2.15
+django==2.2.18
 mako==1.0.2
 requests==2.25.1