Removed django_extensions from example app settings.

[matthewhegarty]

Problem

Whilst testing #1329, I added django_extensions to settings.py so that the runscript command could be used during bulk testing. However, this causes a crash (see issue #1355) if the test.txt dependencies are not used.

Solution

1. Removed django_extensions from settings.
2. Updated bulk testing readme.md to clarify that this needs to be installed if testing.

Acceptance Criteria

• Tested locally in a new virtualenv.
• No new tests necessary.
• Ran the existing test suite.

[coveralls]

Coverage remained the same at 98.073% when pulling 98739b4 on matthewhegarty:remove-django_extensions-from-settings into 1c32c8e on django-import-export:main.