The service SecurityContext is deprectaed in SF 2.6 What is the right service should be used instead of securityContext in Symfony 2.6
As of Symfony 2.6, the SecurityContext class was split in the AuthorizationChecker and TokenStorage classes.
To retreive the user you can use:
$user = $this->get('security.token_storage')->getToken()->getUser();
To check authorization you can use:
if (false === $this->get('security.authorization_checker')->isGranted('ROLE_ADMIN')) { ... }