Fixed Util module name.

I fixed following error. Please merge it. Thanks.

```
$ npm i minify@0.6.3
$ node_modules/.bin/minify public/document.js > public/document.min.js

-
module.js:340
    throw err;
          ^
Error: Cannot find module 'util.io'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at /Users/toshiya/work/syncService/node_modules/minify/bin/minify:10:23
    at Object.<anonymous> (/Users/toshiya/work/syncService/node_modules/minify/bin/minify:58:3)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
```