gistfile1.php

function roots_nav_menu_css_class($classes, $item) {
  $slug = sanitize_title($item->title);
  
  if ($item->menu_item_parent == 0) {
    $active_class = 'active';
  } else {
    $active_class = 'active-child';
  }
  
  $classes = preg_replace('/(current(-menu-|[-_]page[-_])(item|parent|ancestor))/', $active_class, $classes);
  $classes = preg_replace('/^((menu|page)[-_\w+]+)+/', '', $classes);

  $classes[] = 'menu-' . $slug;

  $classes = array_unique($classes);

  return array_filter($classes, 'is_element_empty');
}
add_filter('nav_menu_css_class', 'roots_nav_menu_css_class', 10, 2);