Changed the constant to a protected variable
Changed the constant to a protected variable

This change is due to a problem with the overriding of class constant
which seems unreliable.
This is the reason of the latest updates of Gedmo\Translatable\TranslationListener

felicitus/StofDoctrineExtensionsBundle,IWAtech/StofDoctrineExtensionsBundle,stof/StofDoctrineExtensionsBundle,romaricdrigon/StofDoctrineExtensionsBundle,OskarStark/StofDoctrineExtensionsBundle,jhcasado/StofDoctrineExtensionsBundle,mcfedr/StofDoctrineExtensionsBundle,erikaheidi/StofDoctrineExtensionsBundle,alterphp/StofDoctrineExtensionsBundle