Update slack vendor to master (#958)

vendor/github.com/nlopes/slack/websocket_desktop_notification.go

@@ -0,0 +1,19 @@
+package slack
+
+// DesktopNotificationEvent represents the update event for Desktop Notification.
+type DesktopNotificationEvent struct {
+	Type            string `json:"type"`
+	Title           string `json:"title"`
+	Subtitle        string `json:"subtitle"`
+	Message         string `json:"msg"`
+	Timestamp       string `json:"ts"`
+	Content         string `json:"content"`
+	Channel         string `json:"channel"`
+	LaunchURI       string `json:"launchUri"`
+	AvatarImage     string `json:"avatarImage"`
+	SsbFilename     string `json:"ssbFilename"`
+	ImageURI        string `json:"imageUri"`
+	IsShared        bool   `json:"is_shared"`
+	IsChannelInvite bool   `json:"is_channel_invite"`
+	EventTimestamp  string `json:"event_ts"`
+}