Use correct argument order for single-box-shadow

See [the original function documentation](http://compass-style.org/reference/compass/css3/box_shadow/#mixin-single-box-shadow).

lib/compass/css3/_box-shadow.scss

@@ -50,13 +50,13 @@ $default-box-shadow-inset : false !default;
 }
 
 // Provides a single cross-browser CSS box shadow for Webkit, Gecko, and CSS3.
-// Includes default arguments for color, horizontal offset, vertical offset, blur length, spread length, and inset.
+// Includes default arguments for horizontal offset, vertical offset, blur length, spread length, color and inset.
 @mixin single-box-shadow(
-  $color  : $default-box-shadow-color,
   $hoff   : $default-box-shadow-h-offset,
   $voff   : $default-box-shadow-v-offset,
   $blur   : $default-box-shadow-blur,
   $spread : $default-box-shadow-spread,
+  $color  : $default-box-shadow-color,
   $inset  : $default-box-shadow-inset
 ) {
   @if not ($inset == true or $inset == false or $inset == inset) {