Fix scheme expression parsing

* org-babel-scheme-execute-with-geiser: Do not use elisp reader to parse scheme expression.

Reported by Lawrence Bottorff:

http://lists.gnu.org/archive/html/emacs-orgmode/2016-08/msg00328.html

and also

https://lists.gnu.org/archive/html/emacs-orgmode/2015-09/msg00390.html

lisp/ob-scheme.el

@@ -173,7 +173,7 @@ is true; otherwise returns the last value."
 	(setq result (if (or (string= result "#<void>")
 			     (string= result "#<unspecified>"))
 			 nil
-		       (read result)))))
+		       result))))
     result))
 
 (defun org-babel-execute:scheme (body params)