Taskjuggler export: Target tj3 by default

* contrib/lisp/ox-taskjuggler.el (org-taskjuggler-target-version): The
  default target is now tj3

Since taskjuggler is not developed anymore and hardly any
distributions carry it anymore we ought to target tj3 by default now.

contrib/lisp/ox-taskjuggler.el

@@ -182,7 +182,7 @@ the project."
   :group 'org-export-taskjuggler
   :type 'string)
 
-(defcustom org-taskjuggler-target-version 2.4
+(defcustom org-taskjuggler-target-version 3.0
   "Which version of TaskJuggler the exporter is targeting."
   :group 'org-export-taskjuggler
   :type 'number)