Convert set to list so msgpack and pack it

[DEBUG   ] MasterEvent PULL socket URI: ipc:///var/run/salt/master/master_event_pull.ipc
Process Process-1:
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
      self.run()
        File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/process.py", line 114, in run
            self._target(*self._args, **self._kwargs)
              File "/Volumes/Gob/Data/SaltStack/Code/salt/salt/master.py", line 272, in _clear_old_jobs
                  event.fire_event(data, tagify('present', 'presense'))
                    File "/Volumes/Gob/Data/SaltStack/Code/salt/salt/utils/event.py", line 296, in fire_event
                        event = '{0}{1}{2}'.format(tag, tagend, self.serial.dumps(data))
                          File "/Volumes/Gob/Data/SaltStack/Code/salt/salt/payload.py", line 118, in dumps
                              return msgpack.dumps(msg)
                                File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/msgpack/__init__.py", line 46, in packb
                                    return Packer(**kwargs).pack(o)
                                      File "_packer.pyx", line 206, in msgpack._packer.Packer.pack (msgpack/_packer.cpp:206)
                                        File "_packer.pyx", line 208, in msgpack._packer.Packer.pack (msgpack/_packer.cpp:208)
                                          File "_packer.pyx", line 175, in msgpack._packer.Packer._pack (msgpack/_packer.cpp:175)
                                            File "_packer.pyx", line 203, in msgpack._packer.Packer._pack (msgpack/_packer.cpp:203)
                                            TypeError: can't serialize set(['alpha'])