replace setTimeout with Vue.nextTick & add vue dependency

[lishengzxc]

MessageBox.prompt = function(message, title, options) {
  if (typeof title === 'object') {
    options = title;
    title = '';
  }
  return MessageBox(merge({
    title: title,
    message: message,
    showCancelButton: true,
    showInput: true,
    $type: 'prompt'
  }, options));
};

好像关于prompt的配置，已经设置了showInput: true，感觉不用再去判断this.$els.input了好像~

sorry，刚发现还是应该去判断下的 :)