[AIT-322] Bump protocol version to v6 and add plugin compatibility checks#2191

lawrence-forooghian wants to merge 1 commit intoAIT-324-apply-on-ACKably/ably-cocoa:AIT-324-apply-on-ACKfrom

AIT-322-v6-ProtocolMessageably/ably-cocoa:AIT-322-v6-ProtocolMessageCopy head branch name to clipboard