Crash when receiving invitation to enter a group

[daniele-athome]

From Crashlytics on beta1 (but it's ok since beta2 fixes stuff not related to this):

java.lang.UnsupportedOperationException: Unknown group command: [OWNER_JID];create:[USER1_JID];[USER2_JID];[USER3_JID]
       at org.kontalk.message.GroupCommandComponent.getTextContent(GroupCommandComponent.java:230)
       at org.kontalk.ui.MessagingNotification.updateMessagesNotification(MessagingNotification.java:310)
       at org.kontalk.ui.MessagingNotification$1.run(MessagingNotification.java:166)
       at java.lang.Thread.run(Thread.java:1019)