bug#11166: css-mode can't handle single quote in comments

[Le Wang]

Reproducible with fairly recent Emacs 24 build:

<begin CSS>
        // This should have a white bottom border
        // But the way we're running the main border on tab-content, not on nav-tab 
        // means that this sits under the border line, and so our white border is invisible
        // to fix this, raise the z-index
</end>

Note single quote on line 2 ruins font-locking.

-- 
Le