composer.json fix

Just fixed composer dependency.
Not everybody use "symfony/framework-standard-edition". One can use "symfony/symfony" or something else. So I think it would be better to depend on "symfony/framework-bundle" which is used everywhere, like in other popular bundles:
https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/composer.json

Also removed second "minimum-stability": "dev" as it was written twice in the this file