Merge pull request #363 from e-piksel/v1.5.5.1

Default invoice prefix has been changed
Update upload/install/model/upgrade.php

upgrade fix
Update upload/install/controller/upgrade.php
Update upload/install/model/upgrade.php

change insert to replace to avoid dupicate entries
Update upload/system/library/language.php

to stop the system exiting when a language file can not be found