v0.16.0

• Added support for Django 2.1 and 2.2
• Flushing cache after modifying flags' groups and users
• Removed redundant log line
• Corrected version in waffle/__init__.py
• Fixed bug in tests
• Using strings as cache keys instead of bytes
• Passing effects of test decorator to child classes
  • NOTE: This introduced a backwards-incompatible change for the testutils override decorators.
    See #331 for details.