Fix use of non existing ->getIO() function

src/Composer/EventDispatcher/EventDispatcher.php

@@ -179,7 +179,7 @@ class EventDispatcher
             } else {
                 $args = implode(' ', array_map(array('Composer\Util\ProcessExecutor','escape'), $event->getArguments()));
                 if (0 !== ($exitCode = $this->process->execute($callable . ($args === '' ? '' : ' '.$args)))) {
-                    $event->getIO()->write(sprintf('<error>Script %s handling the %s event returned with an error</error>', $callable, $event->getName()));
+                    $this->io->write(sprintf('<error>Script %s handling the %s event returned with an error</error>', $callable, $event->getName()));
 
                     throw new \RuntimeException('Error Output: '.$this->process->getErrorOutput(), $exitCode);
                 }