Make sure iframe ready event gets fired in all browsers

1. It turns out in Firefox, iframe doesn't fire ready event when its style attribute is `display:none`. I had to change it to `visibility:hidden`.

http://stackoverflow.com/questions/20927992/youtube-api-does-not-appear-to-load-in-firefox-iframe-gets-loaded-but-the-onp

2. `package.js` was missing `gulp-util`, so running `gulp build` kept on throwing this error message
```
Error: Cannot find module 'gulp-concat'
    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 Object.<anonymous> (/Users/jkim/projects/javascript/rhapsody.js/gulpfile.js:2:14)
    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)
    at Module.require (module.js:364:17)
```
Now it's added.