Added Django 2.1 support.

[felixxm]

No description provided.

[felixxm]

Test failure is not related with this PR.

[carltongibson]

Grrr. Still fails if I re-run it. @rpkilby Do you have a spare cycle?

Ref #6073 (comment):

git push -f https://github.com/felixxm/django-rest-framework HEAD:django-2.1

You may not need the -f. Then it's remote_url local_ref:remote_ref.

[rpkilby]

Thanks @felixxm. Superseded by #6141.