Update django-waffle to 0.15.1

[pyup-bot]

This PR updates django-waffle from 0.14.0 to 0.15.1.

Changelog

0.15.1

=======
- Optionally logging missing flags, samples, and switches
- Added --users option to waffle_flag management command
- Updated testutils to flush caches
- Improved admin site for users and groups
- Fixed global cache thread safety issues

0.15.0

=======
- Dropped support for Django < 1.11
- Dropped support for Python 3.3
- Added settings `WAFFLE_CREATE_MISSING_(FLAG|SWITCHES|SAMPLES)` to optionally 
create missing objects in the database automatically
- Allow serializing/de-serializing waffle models by natural key
- Added pluggable Flag model

Links

• PyPI: https://pypi.org/project/django-waffle
• Changelog: https://pyup.io/changelogs/django-waffle/
• Repo: http://github.com/django-waffle/django-waffle

[eviljeff]

looks like we have a failure in one of our ES tests: https://travis-ci.org/mozilla/addons-server/jobs/494221610#L1651

[diox]

Can you do it inside create_switch, create_flag and create_sample instead, flushing every time ?

For the record this is an interesting problem: waffle does flush itself in save(), but it does that on_commit, and inside tests the transaction hasn't been committed at that point... It wouldn't happen if we were only using their test utils but we had issues with those in the past.

[eviljeff]

I was initially confused why we didn't see the same problem with every test that had to manipulate a waffle switch but it's actually pretty rare* for us to actually create a switch (and then want to toggle it) rather than just using override_switch.

*13 times vs 106