fixed autoloader when Swiftmailer is not installed

autoload.php.dist

@@ -22,5 +22,7 @@ $loader->registerPrefixFallbacks(array(
 ));
 $loader->register();
 
-require_once __DIR__.'/vendor/swiftmailer/lib/classes/Swift.php';
-Swift::registerAutoload(__DIR__.'/vendor/swiftmailer/lib/swift_init.php');
+if (is_file(__DIR__.'/vendor/swiftmailer/lib/classes/Swift.php')) {
+    require_once __DIR__.'/vendor/swiftmailer/lib/classes/Swift.php';
+    Swift::registerAutoload(__DIR__.'/vendor/swiftmailer/lib/swift_init.php');
+}