Update ob-ruby for inf-ruby 2.5

 * lisp/ob-ruby.el (org-babel-ruby-initiate-session): `run-ruby` has
   very different semantics in inf-ruby 2.5. Set ruby command to the
   default which used to be set by `run-ruby` and try and find already
   existing buffer before invocation.

lisp/ob-ruby.el

@@ -1,6 +1,6 @@
 ;;; ob-ruby.el --- Babel Functions for Ruby          -*- lexical-binding: t; -*-
 
-;; Copyright (C) 2009-2016 Free Software Foundation, Inc.
+;; Copyright (C) 2009-2017 Free Software Foundation, Inc.
 
 ;; Author: Eric Schulte
 ;; Keywords: literate programming, reproducible research
@@ -150,12 +150,16 @@ If there is not a current inferior-process-buffer in SESSION
 then create one.  Return the initialized session."
   (unless (string= session "none")
     (require 'inf-ruby)
-    (let ((session-buffer (save-window-excursion
-			    (run-ruby nil session) (current-buffer))))
+    (let* ((cmd (cdr (assoc inf-ruby-default-implementation
+			    inf-ruby-implementations)))
+	   (buffer (get-buffer (format "*%s*" session)))
+	   (session-buffer (or buffer (save-window-excursion
+					(run-ruby cmd session)
+					(current-buffer)))))
       (if (org-babel-comint-buffer-livep session-buffer)
 	  (progn (sit-for .25) session-buffer)
-        (sit-for .5)
-        (org-babel-ruby-initiate-session session)))))
+	(sit-for .5)
+	(org-babel-ruby-initiate-session session)))))
 
 (defvar org-babel-ruby-eoe-indicator ":org_babel_ruby_eoe"
   "String to indicate that evaluation has completed.")