[WIP] Symfony 2.3 upgrade

Blockers...

* [x] Memory leak in promotions form. Any action involving ``PromotionType`` kills the process.
* [x] Update the inventory state after completing the order.
* [x] Few validation messages are not translating properly, for Product and TaxRate models.
* [ ] "Too many connections" during Behat database purge, nothing changed here and did not happen on 2.2.
* [ ] Random memory leak during scenarios run.
* [x] Cart currency setting. (Currently hardcoded in model)
* [x] Product prototypes handling, reflecting the bundles split.

Except the random errors/leaks, these issues are quite simple to fix/implement, harder to debug.
Any help with the more problematic issues (where forms steal our memory?!) is greatly appreciated. I think we all want this to be completed, because it makes Sylius customization friendly beast!