Upgrade Symfony to 2.7.49
Upgrade Symfony to 2.7.49

To harden against CVE-2018-14773. In the process Twig was also upgraded
to satisfy Symfony's 2.7.49 requirements.

OpenConext/Stepup-Middleware,SURFnet/Stepup-Middleware,SURFnet/Stepup-Middleware,OpenConext/Stepup-Middleware