Honor $topbar-dropdown-link-color

Change link color in topbar dropdowns to honor the variable designed to configure them: 
``` scss
$topbar-dropdown-link-color
```
It was changed originally from a static "#fff" to "$topbar-link-color" here: https://github.com/zurb/foundation/commit/1daa251f954fc7191b12b8a5253f1ffba7bce483