Add Django 2.1 support and improve test coverage generally

[willbarton]

This PR adds support for Django 2.1, and adds some no covers around Django 1.x/2.x conditionals and adds a test for the admin.

[coveralls]

Pull Request Test Coverage Report for Build 30

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.3%) to 100.0%

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Totals
Change from base Build 16:	2.3%
Covered Lines:	753
Relevant Lines:	753

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💛 - Coveralls