Drop support for Django 1.11

[colekettler]

Drops support for end-of-life Django 1.11.

See: https://www.djangoproject.com/weblog/2020/apr/01/bugfix-releases/

azavea/temperate#1396, azavea/climate-change-api#890, and https://github.com/azavea/mub-monitor/issues/801 have been created to indicate that we're dropping support for 1.11 and that projects will need to upgrade to 2.2 in order to continue to receive Django and OS updates.

I've also confirmed that https://github.com/azavea/grout-server and https://github.com/azavea/raster-vision are no longer active and we don't need to create upgrade issues for them.

Resolves #107

---

Testing

See Travis CI build: https://travis-ci.org/github/azavea/docker-django/builds/701402157

[rbreslow]

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       /' _.--.<
         /'     `'
       /' _.---._____
       \.'   ___, .-'`
           /'    \\             .
         /'       `-.          -|-
        |                       |
        |                   .-'~~~`-.
        |                 .'         `.
        |                 |  R  I  P  |
        |                 |  DJANGO   |
        |                 |  1.11     |
         \              \\|           |//
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[colekettler]

😂 😭