Keep state in instance variable for VersionManager().

Previously, state was stored as a class variable for
betterversion.better.VersionManager(). That meant multiple tests were
mutating the same state across multiple VersionManagers, which is why the
tests in test_supported.py were failing ("data leakage") when all tests
were run, but not when they were run by themselves.

Uncommented tests in test_supported.py, now that they are passing.
Convert regular asserts to unittest assert methods.

This makes the tests in test_backend.py and test_celery.py more
consistent with the rest of the test suite. Plus, unittest asserts
give you more detailed error messages when a test fails.
Merge pull request #674 from toffer/fix-better-version-manager

Keep state in instance variable for VersionManager().
Merge pull request #673 from toffer/convert-to-unittest-asserts

Convert regular asserts to unittest assert methods.
Bump theme
Small mkdocs updates