Made accordion-title-background-active value relative to accordion-title-background

Fixed issue https://github.com/zurb/foundation-apps/issues/243
Changed from
```scss
$accordion-title-background-active: lighten($gray-light, 2) !default;
```
to
```scss
$accordion-title-background-active: smartscale($accordion-title-background, 3%);
```
I referred to foundation for sites codebase  and used smartscaling of 3% for active state.
This creates distinction between between hover (5%) and active (3%) states