'decimal' is not allowed

Decimal is not an option for a Form in Symfony and when using it with FormMapper it raises this exception.

```
Uncaught PHP Exception Symfony\Component\Form\Exception\InvalidArgumentException: "Could not load type "decimal"" at /var/www/releases/20140521095534/vendor/symfony/symfony/src/Symfony/Component/Form/FormRegistry.php line 89 {"exception":"[object] (Symfony\\Component\\Form\\Exception\\InvalidArgumentException: Could not load type \"decimal\" at /var/www/releases/20140521095534/vendor/symfony/symfony/src/Symfony/Component/Form/FormRegistry.php:89)"} []
```