PyPI does not provide download counts of packages anymore and shields.io does also.

This PR replace shields.io's badge with the alternative one provided by the PePy project.

See also: [python - Why PyPi doesn't show download stats anymore? - Stack Overflow](https://stackoverflow.com/questions/38102317/why-pypi-doesnt-show-download-stats-anymore)

Add testing for Django 1.11 - 2.1 and drop testing for older versions

Update test redis version to latest stable: 4.0.11

Add a warning about databases

Add more information to Changelog in README.

Add information about 1.7.1, all 1.6.x releases, and all 1.5.x bugfix
releases.

Fix #105.

Add license to setup.py

This makes it easy to see the license at-a-glance from services such as pyup.io

Drop support for Django < 1.8 and related cleanups

Added DummySerializer to use when values to get/set are already seria…

The need arose when I wanted to cache serialized output of django-tastypie list endpoints. The output was json already, so it makes no sense to re-serialize it with pickle parser or json parser.

Use default timeout in set_many.