Update `apple-touch-icon-precomposed.png` icon size to `180×180`

From: http://www.mobilexweb.com/blog/safari-ios8-iphone6-web-developers-designers:

> A new icon size is available for iPhone 6 Plus, 180×180. iPhone 6 will use the same 120×120 that you should be using today for iOS 7 and (iPhone 4/5)s.

> ```html
<link rel="apple-touch-icon-precomposed" sizes="180x180" href="retinahd_icon.png">
```

--

Can anyone confirm this or provide a link to an official Apple documentation?

(Cc: @mathiasbynens)