Messaging API - August 2020 update

[tkgauri]

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

• Unsend event has been added
• 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
• Video viewing complete event has been added

[kimoto]

Audience API: #183

[Yang-33]

https://github.com/line/line-bot-sdk-ruby/milestone/1 resolves it. Please wait for the release.