prepare 6.4.2 release

[eli-darkly]

[6.4.2] - 2018-09-21

Fixed:

• In polling mode, if the client received an HTTP error from LaunchDarkly, it stopped polling. This has been fixed so it only stops polling if the error is 401 (indicating an invalid SDK key).
• When using a Redis feature store, if the hgetall method returned an invalid result, all_flags and all_flags_state would throw an exception. Instead, all_flags will now return an empty dict, and all_flags_state will return a state object with no flags and valid==False. (Thanks, thieman!)