Fix smarty-gettext#18: return int in execute()

Here is a simple fix that bypass the issue reported in smarty-gettext#18

src/Console/Command/Extract.php

@@ -58,7 +58,7 @@ protected function configure()
     /**
      * @param InputInterface $input
      * @param OutputInterface $output
-     * @return void
+     * @return int
      * @throws FileNotReadableException
      * @throws FileNotWritableException
      * @throws InvalidArgumentException
@@ -92,6 +92,7 @@ protected function execute(InputInterface $input, OutputInterface $output)
         }
 
         $output->writeln('<info>Done</info>');
+        return 0;
     }
 
     /**