Fix parse error thrown in PHP5.5+

When running composer update the file generated by AutoloadGenerator was
not able to be parsed by php due to a require statement inside the foreach
loop.

The fix is to make the statement work the same as the
autoload_namespaces.php require is done.

Issue occured using the following php versions

PHP 5.5.1-2+debphp.org~precise+2
PHP 5.5.3-1+debphp.org~precise+2

src/Composer/Autoload/AutoloadGenerator.php

@@ -460,9 +460,10 @@ REGISTER_AUTOLOAD;
 REGISTER_LOADER;
 
         if ($useIncludeFiles) {
-            $file .= <<<INCLUDE_FILES
+            $file .= <<<'INCLUDE_FILES'
-        foreach (require __DIR__ . '/autoload_files.php' as \$file) {
+        $includeFiles = require __DIR__ . '/autoload_files.php';
-            require \$file;
+        foreach ($includeFiles as $file) {
+            require $file;
         }