Fix symlinking vagrant inventory

The default path assumes that the vagrant dir is called 'inventory'.
With custom defined inventory dirs that are not called 'inventory' this
fails to create the correct symlink under .vagrant.d.

Vagrantfile

@@ -61,7 +61,7 @@ if ! File.exist?(File.join(File.dirname($inventory), "hosts"))
                        "provisioners", "ansible")
   FileUtils.mkdir_p($vagrant_ansible) if ! File.exist?($vagrant_ansible)
   if ! File.exist?(File.join($vagrant_ansible,"inventory"))
-    FileUtils.ln_s($inventory, $vagrant_ansible)
+    FileUtils.ln_s($inventory, File.join($vagrant_ansible,"inventory"))
   end
 end