[Form] [Validator] Form validation not working with 2.5 api

With SF 2.5.3-DEV (commit 98c0621), if `framework.validation.api` = `2.5`, all form validations throw `BadMethodCallException` : 

`
validate() is not supported anymore as of Symfony 2.5. Please use getValidator() instead or enable the legacy mode.
`

Eg: In AcmeDemoBundle, contact action (_demo_contact route name) throws this exception.
**But it works with SF 2.5.2**.
