async changeAnnouncement (id, title, content) {
    const result = await db('announcements').where('id', id).update({
      title,
      content,
      updateTime: (new Date()).getTime()
    })

    return result
  }
Change announcement

@param {Int} id announcement id
@param {String} title announcement title
@param {String} content announcement content

@returns {Int}