telepathyClient: Use a single update for text and timestamp

Commit c6f22826 added a call to update() to refresh the notification
time for sent and received messages. However as it omits the bannerMarkup
parameter, escaped text like ''' now makes it through verbatim.
Rather than adding the parameter, we can just refresh the timestamp in
the existing update() call to fix the issue - it means the notification
timestamp is only refreshed for received messages, but that reflects
the text shown in the notification, which isn't updated for sent
messages either.

https://bugzilla.gnome.org/show_bug.cgi?id=779435