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.
Merge pull request #674 from toffer/fix-better-version-manager

Keep state in instance variable for VersionManager().