Cleanerversion

- Documentation updates
- Slight adaptions of admin tests

Cleanerversion now comes with support for Django 2.0.

For bugs or extension suggestions, please open an issue or a Pull Request right away.

Support for EOL versions of Django (1.9 and 1.10) has officially been dropped.

Several bugs in the admin view are now fixed.

Credits to rossmechanic for contributing the fixes.

With version 2 of CleanerVersion, we support the overhauled ORM that came with Django 1.9.

By taking over all existing tests, we ensured feature-equivalence with former versions of CleanerVersion.

Please note, that version 2.x of CleanerVersion supports Django 1.9+, but not Django 1.8 and below.

Have fun.

This release fixes an issue occurring when prefetching related objects with differing timestamps, as described in 129. Credits go to brki
Additionally, testing was optimized. Thanks to cclauss.

Previously, it was impossible to use `only()` or `defer()` on a QuerySet for a Versionable Model; doing so resulted in an infinite loop.

This has been corrected, and some documentation about the limitations of this were added (specifically, it is not possible to clone or restore a version that has been fetched with deferred fields).