Change "sr" folder name for the two Serbian language packs

[scootergrisen]

Maybe the two language packs for Serbian should not be using the same folder name "sr".
Because if you want to have both language packs at the same time and try to merge the folders into the phpBB folders they will overwrite eachothers files.

Not should sr should be changed to. Maybe sr-latin and sr-cyrl or whatever is correct.

If you
Serbian (Cyrillic Script) / Српски
Serbian (Latin Script) / Srpski (Latinica)