Needs to update composer.lock due to security check

[garak]

Similar to #851
I guess we need to update to latest 4.2.* version of Symfony.

Current output after install:

Executing script security-checker security:check [KO]
 [KO]
Script security-checker security:check returned with error code 1
!!  Symfony Security Check Report
!!  =============================
!!  
!!  2 packages have known vulnerabilities.
!!  
!!  symfony/form (v4.2.0)
!!  ---------------------
!!  
!!   * [CVE-2018-19789][]: Temporary uploaded file path disclosure
!!  
!!  symfony/security-http (v4.2.0)
!!  ------------------------------
!!  
!!   * [CVE-2018-19790][]: Open Redirect Vulnerability on login
!!  
!!  [CVE-2018-19789]: https://symfony.com/cve-2018-19789
!!  [CVE-2018-19790]: https://symfony.com/cve-2018-19790
!!  
!!  Note that this checker can only detect vulnerabilities that are referenced in the SensioLabs security advisories database.
!!  Execute this command regularly to check the newly discovered vulnerabilities.
!!  
Script @auto-scripts was called via post-install-cmd

[javiereguiluz]

This should be fixed in master by #926 and we'll release soon a new stable version. Thanks for reporting!