fix issue for ConfigParser.NoOptionError: No option 'urlfilter' in section: 'pac'

更新 goagent 后出现错误 ConfigParser.NoOptionError: No option 'urlfilter' in section: 'pac'
参考列表
* http://code.google.com/p/goagent/issues/detail?id=10319
* https://github.com/goagent/goagent/commit/06a8fb55329a2ee229694037c060e9