Add setting for self-hosted instance of Voice Chat API

  * HUBOT_VOICECHAT_HOST is an optional argument that should point to your self-hosted instance of the Voice Chat API.
  * HUBOT_VOICECHAT_EXPIRE_CONFERENCE is an optional boolean argument that decides whether or not to show the note about conference rooms expiring after 24 hours.
  * Also added logging in case the request fails.