Messaging API - August 2020 update

[tkgauri]

News: https://developers.line.biz/en/news/2020/08/04/messaging-api-update-august-2020/

• [x] Unsend event has been added
• [x] The audiences property is now optional instead of required for a certain endpoint
• [ ] Create audience for uploading user IDs (by file)
• [ ] Add user IDs or Identifiers for Advertisers (IFAs) to an audience for uploading user IDs (by file
• [x] Video viewing complete event has been added

[kimoto]

Audience API: https://github.com/line/line-bot-sdk-ruby/pull/183