Support Redmine 2.4.2

Migrating to Redmine 2.4.2 make the plugin crash on update:
```
rake aborted!
no implicit conversion of nil into String
/home/redmine-2.4.2/plugins/redmine_stealth/init.rb:24:in `join'
/home/redmine-2.4.2/plugins/redmine_stealth/init.rb:24:in `block in <top (required)>'
/home/redmine-2.4.2/lib/redmine/plugin.rb:73:in `instance_eval'
/home/redmine-2.4.2/lib/redmine/plugin.rb:73:in `register'
/home/redmine-2.4.2/plugins/redmine_stealth/init.rb:19:in `<top (required)>'
/home/redmine-2.4.2/lib/redmine/plugin.rb:133:in `block in load'
/home/redmine-2.4.2/lib/redmine/plugin.rb:124:in `each'
/home/redmine-2.4.2/lib/redmine/plugin.rb:124:in `load'
/home/redmine-2.4.2/config/initializers/30-redmine.rb:19:in `<top (required)>'
/home/redmine-2.4.2/config/environment.rb:14:in `<top (required)>'
Tasks: TOP => redmine:plugins:migrate => environment
(See full trace by running task with --trace)
```

I have changed the directory variable with the current path of the file.