Fix issue with github ribbon on demo page

The issue (Chrome):
![js-yaml_gh-ribbon_issue](https://cloud.githubusercontent.com/assets/1329477/3842590/8d2e7c68-1e38-11e4-9996-15cfa394a601.png)

So I've updated transform property. Note that `-moz-` prefix no longer needed, it's deprecated since FF 16, see [compatibility table](https://developer.mozilla.org/en-US/docs/Web/CSS/transform#Browser_compatibility).