Add Django 1.11, 2.0 and 2.1 to the build matrix

[browniebroke]

Goal

Django 2.1 was released today, which means all versions of Django in the build matrix are currently no longer supported by the Django core team. This adds newer versions of Django to the build matrix.

You might want to drop support for older versions <1.11 and bump the version of the library. Let me know if you want me to do so as part of this PR, I don't mind it.

Design

n/a

Tests

Running Python 2.7 tests under Django 1.11 but only Python 3.6 on Django 2.0 and above, as the project dropped support for Python 2.

Discussion

Alternative Approaches

Outstanding Questions

Linked issues

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[fractalwrench]

Many thanks for this contribution @browniebroke!