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group_topic message event

A group topic was updated

Compatibility: RTM

    "type": "message",
    "subtype": "group_topic",
    "ts": "1358877455.000010",
    "user": "U2147483828",
    "topic": "hello world",
    "text": "<@U2147483828|cal> set the group topic: hello world"

A group_topic message is sent when the topic for a private group is changed.