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.
Merge pull request #191 from eugeneius/pre_css_class

Add CSS class to pre tag instead of code tag