Update to Django 2.x

[kousu]

Django 1.11-LTS will expire in 15 months:

Sous-Chef needs to be upgraded before then. What's more, this is a major version bump, so there will be a lot of work needed.

There are similar maintenance problems elsewhere in the stack : #834

[erozqba]

@kousu I would like to suggest to focus into an upgrade to the next LTS version when it's available and not to go into 2.0 or 2.1.

[kousu]

@erozqba that would be 2.2, which will be out in ~3 months. I doubt anyone involved in Sous-Chef will find the time to start looking at this before then.

[manumilou]

It's fine to stick to the LTS versions. You'll have until 1.11 gets deprecated to plan the upgrade.