Bump to 4.2.0 release

[jjlawren]

Breaking changes

PlexClient.timeline() previously returned a raw XML object. Now returns a newly available ClientTimeline instance for the current active session. See #572.

New Features

• Edit Library and Show's Advanced Settings #507 - Edit Library and Show's Advanced Settings
• Library Hubs and Music Stations #518 - Library Hubs and Music Stations
• Add download feature to Photo library #582 - Add download feature to Photo library
• Update timeline to return an active timeline object w/ attributes #572 - Update timeline to return an active timeline object w/ attributes
• Allow scanning for clients when using GDM.all() #585 - Allow scanning for clients when using GDM.all()
• Make Photo objects Playable #592 - Make Photo objects Playable

Bug fixes

• Add server handling of HTTP status code 204 #580 - Add server handling of HTTP status code 204
• Handle empty datetimes in tests #581 - Handle empty datetimes in tests
• Updating cleanBundles() and optimize() methods #583 - Updating cleanBundles() and optimize() methods
• Collection fields not pulling locked fields #588 - Collection fields not pulling locked fields
• Don't overwrite existing attributes with an empty list #594 - Don't overwrite existing attributes with an empty list

[coveralls]

Coverage decreased (-5.0%) to 65.408% when pulling f2b1b19 on jjlawren:bump_4.2.0 into 380fd78 on pkkid:master.