Update to Django 1.9 about sprints

[jwhitlock]

Django tests now pass in 1.8 and 1.9. Looking at the release notes, there may be some additional changes:

• 1.9 updates the default LOGGING setting. We may need updates to continue seeing log messages from Django, and to be creative to get one that works in 1.8 and 1.9.

• In the Miscellaneous section, it says:

  If you implement a custom handler404 view, it must return a response with an HTTP 404 status code. Use HttpResponseNotFound or pass status=404 to the HttpResponse. Otherwise, APPEND_SLASH won’t work correctly with DEBUG=False.

  Our customized middlewares may handle this, or it may only appear in staging and production.

• In Features removed in 1.9:

  The zh-cn and zh-tw language codes are removed.

  We plan to continue using zh-CN and zh-TW to match Mozilla's traditional locales, but testing is needed.

• The 1.9.11 release notes include that ALLOWED_HOSTS is validated when settings.DEBUG=True. This shouldn't affect deployments, but might affect development.

We can determine if these are issues by sending a trial deployment of 1.9 to staging, but we may want to continue focusing on Django tests for 1.10 and 1.11, rather than get involved in 1.9 behavior.