Add CSS class to pre tag instead of code tag

Background colours have been broken since `v1.4.0`, as noted here: https://github.com/jneen/rouge/pull/141#issuecomment-45667410

This is because the highlight CSS class is added to the `code` tag, which is an inline element. Adding the class to the surrounding `pre` tag makes the background display correctly, since it's a block element.