Update django-waffle to use clintonb's fix for django 1.8+

Description

Uses @clintonb 's `django-waffle==0.21.0` to fix errors when using `waffle_switch` and `waffle_flag` management commands.

*JIRA tickets*: OSPR

*Discussions*: cf https://github.com/jsocol/django-waffle/pull/244, [jsocol/django-waffle@v0.12.0](https://github.com/jsocol/django-waffle/commits/v0.12.0)

*Partner information*: 3rd party hosted

*Deployment targets*: insights.edx.org, insights.stage.edx.org

*Merge deadline*: `open-releae/ginkgo.rc1`, if possible cc @nedbat

*Testing instructions*:

1. Deploy Insights on an analytics devstack using the current master.
1. Try listing the waffle flags:
```
sudo su insights
./manage.py waffle_switch --list --settings=analytics_dashboard.settings.production
```
Note `AttributeError: type object 'BaseCommand' has no attribute 'option_list'`
1. Checkout the branch from this PR: `jill/update-django-waffle`
1. Install the pip requirements to upgrade django-waffle:
```
make requirements.py
```
1. Re-run the waffle_switch command from step 2, and note success.

*Reviewers*

  • [ ] OpenCraft reviewer TBD

  • [ ] edX reviewers TBD - @edx/analytics CC @thallada

Done

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Contributor Name

Jillian Vogel

Repo

edx/edx-analytics-dashboard

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

edX Theme

None

edX Squad

None

Github Lines Added

None

Github Lines Deleted

None

Priority

Unset