EZP-20422 - router_proxy configuration has been renamed to fragments

Hi. As explained in jira, it seems router_proxy has been renamed. 
see https://github.com/symfony/FrameworkBundle/commit/bbede10727830df6bf1e116216183accd4c5f579

Because of this, and as router_proxy is in the config.yml and also enabled, i was having errors because that entry is not recognized now.