core: Replace CACHE_DIR by PATH_CACHE

Move CACHE_DIR from index.php to /lib/RssBridge.php and change name
to PATH_CACHE.

PATH_CACHE is one of the core paths of RSS-Bridge and should therefore
be defined in the core file RssBridge.php.

lib/RssBridge.php

@@ -2,6 +2,7 @@
 
 define('PATH_VENDOR', __DIR__ . '/../vendor'); // Path for vendor library
 define('PATH_LIB', __DIR__ . '/../lib'); // Path for core library
+define('PATH_CACHE', __DIR__ . '/../cache'); // Path to cache folder
 
 // Classes
 require_once PATH_LIB . '/Exceptions.php';