Change configurations.setup() to importer.install()

[ewelkaw]

Trying to run example from the docs with Celery results in error message:

django.core.exceptions.ImproperlyConfigured: django-configurations settings importer wasn't correctly installed. Please use one of the starter functions to install it as mentioned in the docs: https://django-configurations.readthedocs.io/

However, changing that to:

from configurations import importer
importer.install()

works fine.

This PR updates the documentation to provide correct command.

[codecov]

Codecov Report

Merging #292 (2c39c41) into master (d89fe5a) will not change coverage.
The diff coverage is n/a.

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  Lines        1198     1198           
  Branches       98       98           
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  Hits         1077     1077           
  Misses         92       92           
  Partials       29       29           

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[brianhelba]

@ewelkaw I'm not able to reproduce this myself.

Internally, the only difference is that configurations.setup() calls django.setup() too. However, Celery also calls django.setup() internally, since doing so is required in order to use Django settings.

If there's a good reason for this change, I'd be happy to consider it, but configurations.setup() is generally the preferred high-level way to bootstrap things, so I don't think we should change the docs unless it's clearly understood why it's necessary.