compatibility with coffeescript 1.7.1

修复 coffeescript 兼容性问题, coffeescript  ~ <= 1.7.1  验证通过

child_process.fork 返回的对象 pChild 的 stdin, stdout, stderr 值均为 null.

错误信息如下:
```
E:\code\fedproj>fed server -w ./configs/index.json
TypeError: Cannot call method 'on' of null
  at launchServer (C:\Users\bitdewy\AppData\Roaming\npm\node_modules\fed\libs\m
odules\server\index.coffee:57:16)
  at exports.exec (C:\Users\bitdewy\AppData\Roaming\npm\node_modules\fed\libs\m
odules\server\index.coffee:23:2)
  at Object.<anonymous> (C:\Users\bitdewy\AppData\Roaming\npm\node_modules\fed\
fed.coffee:41:9)
  at Object.<anonymous> (C:\Users\bitdewy\AppData\Roaming\npm\node_modules\fed\
fed.coffee:4:1)
  at Module._compile (module.js:456:26)

E:\code\fedproj>
```