Allow update.php to be run from outside of the main directory

* Change the update.php working directory to be the main tt-rss application
   directory so that the script can be run from anywhere without problems.
   Useful for adding the update.php script to cron.

update.php

@@ -2,6 +2,7 @@
 <?php
 	define('DISABLE_SESSIONS', true);
 
+	chdir(dirname($_SERVER['SCRIPT_NAME']));
 	require_once "functions.php";
 	require_once "sanity_check.php";
 	require_once "config.php";