config.yml

parameters:
    kernel.include_core_classes: false

kernel.config:
    charset:             UTF-8
    error_handler_level: null

web.config: ~

web.templating:
    escaping:       htmlspecialchars
    assets_version: SomeVersionScheme

# as documented in http://symfony-reloaded.org/guides/Validator
web.validation:
    annotations: true

doctrine.dbal:
    driver:     PDOMySql
    dbname:     dbname
    user:       root
    password:   root

doctrine.orm: ~

swift.mailer:
    transport: gmail
    username:  xxxxxxxx
    password:  xxxxxxxx

The error is :
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'The tag "web:validation" is not defined in the "web" extension.'

And with :
web:
validation: true
I have :
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'The "web" tag is not valid