Breaking changes

• Dropped support for Node.js < 4.5.0.
• The new operator is now required to create all instances as we moved to ES6
  classes.
• Error messages have been simplified.
• The clients property of the WebSocketServer is no longer an Array but a
  Set and is only set if the clientTracking option is truthy (#806).
• The default HTTP status message is now used when handshake fails (41e7cae).
• Removed support for the Hixie-76 version of the protocol (#871).
• Removed ability to specify different paths for multiple WebSocketServers
  when binding them to the same underlying HTTP/s server (#885).
• Removed WebSocket.prototype.stream() and ability to pass a readable stream
  to WebSocket.prototype.send() (#875).
• Removed callback argument from handleProtocols handler (#890).
• Removed supports property from WebSocket (#918).
• Removed WebSocket.createServer(), WebSocket.createConnection(), and
  WebSocket.connect() factory functions (#926).
• The second argument of WebSocket.prototype.ping() and
  WebSocket.prototype.pong() is no longer an options object but a boolean
  (#951).
• An error is emitted if WebSocket.prototype.close() is called before the
  connection is established (#956).

The following breaking changes only apply if you required the mentioned classes
directly.

• Removed Sender inheritance from EventEmitter (#861).
• Removed BufferPool class (73ab370).
• Made extensions a required argument for the Receiver constructor (5f53194).
• receiver.onbinary and receiver.ontext have been merged into
  receiver.onmessage (#939).

Features

• Added ability to set TCP backlog for WebSocketServer (#795).
• Added checkServerIdentity option to WebSocket (#701).
• Added a threshold option for permessage-deflate to only compress messages
  whose size is bigger than threshold (6b3904b).
• Added shouldHandle method to WebSocketServer to see if a request should
  be accepted or rejected. This method can be overridden by the user if a
  custom logic is desired (6472425).
• Added removeEventListener method to WebSocket (078e96a).
• Added family option to WebSocket (#962).

Bug fixes

• Fixed an issue that prevented permessage-deflate options from being correctly
  handled (#744).
• All error events are now emitted with a proper Error instance (#789).
• Fixed an issue that could cause a stack overflow crash (#810).
• Added 1012 and 1013 to the list of allowed close codes (b58f688).
• Fixed an issue that prevented the connection from being closed when path
  validation failed (#534).
• Fixed an issue where the fin option of WebSocket.prototype.send() was
  unconditionally set to true (ea50be7).
• Fixed an issue that prevented the total length of a fragmented message from
  being correctly calculated (545635d).
• Fixed an issue where zlib.flush() was called with a wrong flush level
  (#733).
• The callback of WebSocketServer.prototype.close() is now invoked when the
  close event is emitted by the underlying HTTP/s server (#892).
• Fixed an issue that prevented the server from listening on IPv6 addresses
  with default settings (dcdc652).
• Fixed an issue where the connection event was emitted even if the client
  closed the connection during the handshake process (04530ad).
• The masking key is now generated using crypto.randomBytes() instead of
  Math.random() (7253f06).
• Fixed an issue that, under particular circumstances, caused data to be
  discarded (#945).
• Fixed an issue that prevented clients from being removed from the clients
  set (#955).
• WebSocket.prototype.close() now works as expected if called on the client
  before the connection is established (#956).
• WebSocket.prototype.send() no longer mutates the options object (#968).
• The bufferedAmount getter now takes into account the data queued in the
  sender (#971).