Update django requirement from <2.2 to <2.3 (#110)

Updates the requirements on [django](https://github.com/django/django) to permit the latest version.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.0...2.2.4)

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

requirements-docs.txt

@@ -1,4 +1,4 @@
 -r requirements-test.txt
-Django<2.2
+Django<2.3
 djangocms-helper
 djangocms-page-tags>0.6,<0.7