Update extensions.php

Fix of "NOTICE: PHP message: PHP Notice:  Array to string conversion in /.../admin/extensions.php on line 919" in PHP 5.4

http://punbb.ru/topic5405-notice-php-message-php-notice-array-to-string-conversion.html

admin/extensions.php

@@ -915,7 +915,7 @@
 		$update_hour = (isset($forum_ext_versions_update_cache) && (time() - $forum_ext_versions_update_cache > 60 * 60));
 
 		// Update last versions if there is no cahe or some extension was added/removed or one day has gone since last update
-		$update_new_versions_cache = !defined('FORUM_EXT_VERSIONS_LOADED') || (isset($forum_ext_last_versions) && array_diff($inst_exts, $forum_ext_last_versions) != array()) || $update_hour || ($update_hour && isset($min_timestamp) && (time() - $min_timestamp > 60*60*24));
+		$update_new_versions_cache = !defined('FORUM_EXT_VERSIONS_LOADED') || (isset($forum_ext_last_versions) && array_diff(array_keys($inst_exts), array_keys($forum_ext_last_versions)) != array()) || $update_hour || ($update_hour && isset($min_timestamp) && (time() - $min_timestamp > 60*60*24));
 
 		($hook = get_hook('aex_before_update_checking')) ? eval($hook) : null;