fix incomplete clearing of RDY timeout reference

fixes:

```
[I 130624 16:03:34 Reader:578] [host:4150:topic:channel] redistributing RDY
[E 130624 16:03:34 ioloop:419] Error in periodic callback
    Traceback (most recent call last):
      File "/usr/local/lib/python2.5/site-packages/tornado/ioloop.py", line 415, in _run
        self.callback()
      File "/usr/local/lib/python2.5/site-packages/nsq/Reader.py", line 579, in _redistribute_rdy_state
        self._send_rdy(conn, 1)
      File "/usr/local/lib/python2.5/site-packages/nsq/Reader.py", line 361, in _send_rdy
        self.ioloop.remove_timeout(conn.rdy_timeout)
      File "/usr/local/lib/python2.5/site-packages/tornado/ioloop.py", line 322, in remove_timeout
        self._timeouts.remove(timeout)
    ValueError: list.remove(x): x not in list
```

cc @jehiah