Update codebase for Django 3 and Python 3.8

[WayneLambert]

Upgrades to tox.ini, travis.yml and setup.py to run tests against Python 3.8 and Django 3.0

Also, the version number has been bumped in CHANGELOG.rst and my name added in CONTRIBUTORS.md

[coveralls]

Pull Request Test Coverage Report for Build 986

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.599%

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Totals
Change from base Build 983:	0.0%
Covered Lines:	645
Relevant Lines:	728

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💛 - Coveralls

[WayneLambert]

Please can you review the pull request? Feel free to make any changes as necessary in order for compatibility with Django 3.0 and Python 3.8.

Does the package's source code or test suite configuration need any further work since Django 3.0 will no longer support Python 2? Some of the failed Travis CI tests seem to be Python 2 specific.

I trust comparing straight to master is OK since there is nothing specific within the contribution guidelines to say otherwise?

[atodorov]

Rebase to pick up the changes from #260 and squash your commits together.

[carlio]

This would also need a rebase / merge with master if you could @WayneLambert