Update format-time-string calls for argument change

* lisp/org.el (org-timestamp-format):
* lisp/ox-icalendar.el (org-icalendar-convert-timestamp): Specify t for
  the ZONE argument of format-time-string to indicate UTC.

As of Emacs's af32fa9 (New optional ZONE arg for format-time-string
etc., 2015-07-26), UTC is indicated by passing t as the third argument
to format-time-string rather than any non-nil value.

This change is in place of a backport of Emacs's b787d55 (More
format-time-string change fixups, 2016-04-02), which would not work for
earlier versions of Emacs.

lisp/org.el

@@ -23813,7 +23813,8 @@ time-range, if possible.
 When optional argument UTC is non-nil, time will be expressed as
 Universal Time."
   (format-time-string
-   format (org-timestamp--to-internal-time timestamp end) utc))
+   format (org-timestamp--to-internal-time timestamp end)
+   (and utc t)))
 
 (defun org-timestamp-split-range (timestamp &optional end)
   "Extract a TIMESTAMP object from a date or time range.

lisp/ox-icalendar.el

@@ -396,7 +396,8 @@ Universal Time, ignoring `org-icalendar-date-time-format'."
       ;; Convert timestamp into internal time in order to use
       ;; `format-time-string' and fix any mistake (i.e. MI >= 60).
       (encode-time 0 mi h d m y)
-      (or utc (and with-time-p (org-icalendar-use-UTC-date-time-p)))))))
+      (and (or utc (and with-time-p (org-icalendar-use-UTC-date-time-p)))
+	   t)))))
 
 (defun org-icalendar-dtstamp ()
   "Return DTSTAMP property, as a string."