Update isotope.js

Please use this notation  ['./item.js', './layout-modes.js'] to keep proper relative paths, when loading via requireJS. If not it tries to load item.js and layout-modes.js from configured baseUrl
 ```javascript
require.config({
  baseUrl: 'js'
});
```

Similar issue/solution explained here <http://stackoverflow.com/questions/14548369/requirejs-relative-paths/14589782#14589782>