ox-beamer: Wrap labels in braces

* ox-beamer.el (org-beamer--get-label): Wrap labels in braces.

The colon in the label name causes an argument parsing problem, for
more see <http://tex.stackexchange.com/q/250640/4416>.

lisp/ox-beamer.el

@@ -337,7 +337,7 @@ function assumes HEADLINE will be treated as a frame."
     (if (and (stringp opt)
 	     (string-match "\\(?:^\\|,\\)label=\\(.*?\\)\\(?:$\\|,\\)" opt))
 	(match-string 1 opt)
-      (concat "sec:" (org-export-get-reference headline info)))))
+      (format "{sec:%s}" (org-export-get-reference headline info)))))
 
 (defun org-beamer--frame-level (headline info)
   "Return frame level in subtree containing HEADLINE.