Update composer.json - FOSUser master branch is in 2.0 version

Last modifications are dependants of the master fos user bundle which is the 2.0 version, not the 1.3 any more.

By example :

https://github.com/sonata-project/SonataUserBundle/blob/master/Controller/RegistrationController.php#L46

Not compatible with 1.3 : 
https://github.com/FriendsOfSymfony/FOSUserBundle/blob/v1.3.3/Controller/RegistrationController.php#L30

Compatible with master (2.0) : 
https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Controller/RegistrationController.php#L34