Fix PHP requirement in installation documentation

See https://github.com/symfony/symfony-docs/pull/3198
Merging #354
Merging #352
Merging #355
Update checkbox.rst // no checked option

Salut !

En comparant avec la version anglaise
http://symfony.com/doc/master/reference/forms/types/checkbox.html on remarque que l'option checked n'existe pas (plus?).

Je viens de tester le code suivant

$builder->add('_remember_me', 'checkbox',
            array(
                'checked' => true,
                )
            )

Et effectivement, Symfony me renvoie une erreur:
The option "checked" does not exist.
Merging #358
Erreur de conjugaison
Typo in Templating
Add warning on requirements page
Removing PHP version specific information
Typo in Glossary : an extra R removed
Add a warning about PHP 5.3.16 too
Merging #362
Merge pull request #361 from HassanCehef/TypoGlossary

Typo glossary
Merge pull request #360 from HassanCehef/patch-1

Erreur de conjugaison
Merge pull request #357 from gabriel-kaam/patch-1

Update checkbox.rst // no checked option
Merge pull request #351 from gnutix/patch-1

Fix PHP requirement in installation documentation