Update gh-fork-ribbon.css.

* add missing fallback border-color
* remove unit from 0 and change `0.00` to `0`
* add missing vendor prefixed properties
* use the shorthand font property
* fix typo in comment
* use the hex colors consistently
* remove comments for self-explanatory code
index.html: minor cleanup in the inline CSS.
Fix IE 9 issue.

Move `filter` property to gh-fork-ribbon.ie.css and include the css if less than IE 9.

Also make sure we use the latest IE version by setting the `X-UA-Compatible` property.