Fix deprecated class usage

As noted in: https://github.com/composer/composer/releases/tag/1.0.0-alpha10 (April 2015), \Composer\Installer\PackageEvent should be used instead of \Composer\Script\PackageEvent

Author: NickDickinsonWilde

src/Plugin.php

@@ -14,7 +14,7 @@
 use Composer\EventDispatcher\EventSubscriberInterface;
 use Composer\IO\IOInterface;
 use Composer\Plugin\PluginInterface;
-use Composer\Script\PackageEvent;
+use Composer\Installer\PackageEvent;
 use Composer\Script\ScriptEvents;
 use Composer\Util\Filesystem;
 
@@ -55,7 +55,7 @@ public static function getSubscribedEvents()
     /**
      * Pre Package event behaviour for backing up preserved paths.
      *
-     * @param \Composer\Script\PackageEvent $event
+     * @param \Composer\Installer\PackageEvent $event
      */
     public function prePackage(PackageEvent $event)
     {
@@ -66,7 +66,7 @@ public function prePackage(PackageEvent $event)
     /**
      * Pre Package event behaviour for backing up preserved paths.
      *
-     * @param \Composer\Script\PackageEvent $event
+     * @param \Composer\Installer\PackageEvent $event
      */
     public function postPackage(PackageEvent $event)
     {