prepare 6.0.0 release

[eli-darkly]

[6.0.0] - 2018-05-10

Changed:

• To reduce the network bandwidth used for analytics events, feature request events are now sent as counters rather than individual events, and user details are now sent only at intervals rather than in each event. These behaviors can be modified through the LaunchDarkly UI and with the new configuration option inline_users_in_events. For more details, see Analytics Data Stream Reference.
• The analytics event processor now flushes events at a configurable interval defaulting to 5 seconds, like the other SDKs (previously it flushed if no events had been posted for 5 seconds, or if events exceeded a configurable number). This interval is set by the new Config property flush_interval.

Removed:

• Python 2.6 is no longer supported.
• Removed the Config property events_upload_max_batch_size, which is no longer relevant in the new event flushing logic (see above).