Add missing properties in app_mention event payload

Author: seratch

slack-api-model/src/main/java/com/slack/api/model/event/AppMentionEvent.java

@@ -47,6 +47,8 @@ public class AppMentionEvent implements Event {
     private Boolean displayAsBot;
     private String ts;
     private String team;
+    private String userTeam;
+    private String sourceTeam;
     private String channel;
     private Edited edited;
     private String eventTs;