Fixes recent pull request causing $topbar-link-color to be ignored 

A recent pull request (https://github.com/zurb/foundation/pull/1730)
which causes $topbar-link-color to be ignored but
$topbar-dropdown-link-color to be used instead for the topbar link
color.  These changes honor the intention of $topbar-link-color and
$topbar-dropdown-link-color;