Fixes the hardcoded environ name for gunicorn worker

On the `__init__.py` the `socketio_manage()` uses the environ `socketio` but here it was using `socket.io`, this was causing a `KeyError` when using gunicorn.

```
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/gevent/pywsgi.py", line 504, in handle_one_response
    self.run_application()
  File "/usr/local/lib/python2.7/dist-packages/gevent/pywsgi.py", line 490, in run_application
    self.result = self.application(self.environ, self.start_response)
  File "/home/perone/projects/python/gsock.py", line 130, in __call__
    socketio_manage(environ, {'': HappyNamespace}, self.request)
  File "/usr/local/lib/python2.7/dist-packages/socketio/__init__.py", line 60, in socketio_manage
    socket = environ['socketio']
KeyError: 'socketio'
```