Use 127.0.0.1 as proxy IP to prevent error when LISTEN_IP = 0.0.0.0

Update of PAC file is always originated from the same computer where GoAgent is running so using 127.0.0.1 will not cause problem.
![success](https://f.cloud.github.com/assets/3032746/352639/39f140e6-a06a-11e2-9c99-67e1d6e3a156.png)