added UnknownMessage for unrecognized message types

lib/skyfall/messages/unknown_message.rb

···
+module Skyfall
+  class UnknownMessage < WebsocketMessage
+  end
+end

lib/skyfall/messages/websocket_message.rb

···
     require_relative 'commit_message'
     require_relative 'handle_message'
     require_relative 'info_message'
+    require_relative 'unknown_message'
 
     attr_reader :type_object, :data_object
     attr_reader :type, :did, :seq
···
         when '#commit' then CommitMessage
         when '#handle' then HandleMessage
         when '#info' then InfoMessage
-        else WebsocketMessage
+        else UnknownMessage
       end
 
       message = message_class.allocate