Zend Framework fails to detect temp dir.

Sometimes Zend Framework fails to detect temp directory (it uses is_writable() which has some flaws). That's why SS sets these $_ENV variables [here](https://github.com/silverstripe/silverstripe-framework/blob/3.1/_config.php#L37).

The problem is that Fluent::init() is called even before framework/_config.php and you may get an error [here] (https://github.com/tractorcow/silverstripe-fluent/blob/3.1/code/Fluent.php#L383).