change default sh code block shell to bash

* lisp/ob-sh.el (org-babel-sh-command): Change default sh code block
  shell to bash.

lisp/ob-sh.el

@@ -38,7 +38,7 @@
 
 (defvar org-babel-default-header-args:sh '())
 
-(defvar org-babel-sh-command "sh"
+(defvar org-babel-sh-command "bash"
   "Command used to invoke a shell.
 This will be passed to  `shell-command-on-region'")