Use upstream slack-go/slack again (#1018)

vendor/github.com/slack-go/slack/websocket_pins.go

@@ -0,0 +1,16 @@
+package slack
+
+type pinEvent struct {
+	Type           string `json:"type"`
+	User           string `json:"user"`
+	Item           Item   `json:"item"`
+	Channel        string `json:"channel_id"`
+	EventTimestamp string `json:"event_ts"`
+	HasPins        bool   `json:"has_pins,omitempty"`
+}
+
+// PinAddedEvent represents the Pin added event
+type PinAddedEvent pinEvent
+
+// PinRemovedEvent represents the Pin removed event
+type PinRemovedEvent pinEvent