Drupal 8: Add a class to the more link of a view.

/**
 * Implements template_preprocess_views_view()
 * @param array $vars
 */
function MYTHEME_preprocess_views_view(&$variables) {
  if ($variables['id'] == 'MY_VIEW_ID') {
    $variables['more']['#options']['attributes']['class'] = array(
      'button'
    );
  }
}