issue: Flush Model Cache

This addresses an issue where `ModelMeta::flushModelCache()` doesn't work
correctly. In theory, it's supposed to clear all APCu cache. This is
especially helpful for Upgrades by preventing the selection of
data/structures that don't exist anymore. This is due to the fact that when
we updated `apc_clear_cache()` to `apcu_clear_cache()` we forgot to remove
the argument. `apcu_clear_cache()` takes no arguments and you would think
that PHP would be smart enough to void the argument but apparently it's not.
This removes the argument so that it works properly and clears all APCu
cache.

include/class.orm.php

@@ -298,7 +298,7 @@ function inspectFields() {
 
     static function flushModelCache() {
         if (self::$model_cache)
-            @apcu_clear_cache('user');
+            @apcu_clear_cache();
     }
 }