Fix for AttributeError: environ

Seeing this in the logs:

    Traceback (most recent call last):
      File "/usr/local/lib/python2.7/site-packages/gevent/greenlet.py", line 390, in run
        result = self._run(*self.args, **self.kwargs)
      File "/usr/local/lib/python2.7/site-packages/gevent_socketio-0.3.6-py2.7.egg/socketio/server.py", line 124, in handle
        handler.handle()
      File "/usr/local/lib/python2.7/site-packages/gevent/pywsgi.py", line 180, in handle
        result = self.handle_one_request()
      File "/usr/local/lib/python2.7/site-packages/gevent/pywsgi.py", line 314, in handle_one_request
        self.handle_one_response()
      File "/usr/local/lib/python2.7/site-packages/gevent_socketio-0.3.6-py2.7.egg/socketio/handler.py", line 184, in handle_one_response
        del self.websocket.environ
    AttributeError: environ
    <Greenlet at 0x31244b0: <bound method SocketIOServer.handle of <SocketIOServer at 0x3020210 fileno=5 address=127.0.0.1:8027>>(<socket at 0x3016d50 fileno=[Errno 9] Bad file des, ('127.0.0.1', 34349))> failed with AttributeError

I don't know if self.websocket.environ should really be deleted (it's not accesses anywhere in the code), but this PR should at least fix the attribute error.