[Payum] Upgrade up to 0.12 

This PR updates Payum up to 0.12 (it is still in development). With this version it is extremely easy to add a payment gateway.

* Configure `PayumBundle` (follow the Payum's [doc](http://payum.org/doc#PayumBundle))

```yaml
# app/config/config.yml

twig:
    paths:
        %kernel.root_dir%/../vendor/payum/payum/src/Payum/Core/Resources/views: PayumCore
        %kernel.root_dir%/../vendor/payum/payum/src/Payum/Stripe/Resources/views: PayumStripe

payum:
    storages:
        Sylius\Component\Core\Model\Order: { doctrine: orm }
        Sylius\Component\Core\Model\Payment: { doctrine: orm }

    security:
        token_storage:
            Sylius\Bundle\PayumBundle\Model\PaymentSecurityToken: { doctrine: orm }

        stripe_checkout:
            stripe_checkout:
                publishable_key: %stripe.publishable_key%
                secret_key: %stripe.secret_key%
```

* Add `stripe_checkout` to sylius_payment_methods table.
* Add `stripe_checkout` to 

```yaml
# app/config/config.yml

sylius_payment:
    gateways:
        stripe_checkout: Stripe Checkout
```

That's it. All other [payments supported](http://payum.org/doc/0.12/Core/supported-payments) by Payum could be added similar way.