_animateOut(index) {
      let $collapsibleLi = this.$el.children('li').eq(index);
      if ($collapsibleLi.length) {
        let $body = $collapsibleLi.children('.collapsible-body');
        anim.remove($body[0]);
        $body.css('overflow', 'hidden');
        anim({
          targets: $body[0],
          height: 0,
          paddingTop: 0,
          paddingBottom: 0,
          duration: this.options.outDuration,
          easing: 'easeInOutCubic',
          complete: () => {
            $body.css({
              height: '',
              overflow: '',
              padding: '',
              display: ''
            });

            // onCloseEnd callback
            if (typeof this.options.onCloseEnd === 'function') {
              this.options.onCloseEnd.call(this, $collapsibleLi[0]);
            }
          }
        });
      }
    }
Animate out collapsible slide
@param {Number} index - 0th index of slide to open