Make [fragile] work with overlay specifications.

* org-beamer.el (org-beamer-auto-fragile-frames): Make
[fragile] work with overlay specifications.

If the BEAMER_envargs property contains optional parameters together
with an overlay specification like [option]<1-2>, the exporter turns
the start of the frame into \begin{frame}<1-2>[option].  If then
[fragile] needs to be added, this becomes
\begin{frame}[fragile]<1-2>[option] and causes a LaTeX error.

With this patch [fragile] is added in a way such that the line becomes
\begin{frame}<1-2>[fragile,option].

TINYCHANGE

lisp/org-beamer.el

@@ -503,7 +503,7 @@ This function will run in the final LaTeX document."
       (while (re-search-forward org-beamer-fragile-re nil t)
 	(save-excursion
 	  ;; Are we inside a frame here?
-	  (when (and (re-search-backward "^[ \t]*\\\\\\(begin\\|end\\){frame}"
+	  (when (and (re-search-backward "^[ \t]*\\\\\\(begin\\|end\\){frame}\\(<[^>]*>\\)?"
 					 nil t)
 		     (equal (match-string 1) "begin"))
 	    ;; yes, inside a frame, make sure "fragile" is one of the options