Security update to 11.5.4.
See here for more details:
https://about.gitlab.com/2018/12/13/critical-security-release-gitlab-11-dot-5-dot-4-released/

Approved by: mentors (implicit)
MFH: 2018Q4
Security: 757e6ee8-ff91-11e8-a148-001b217b3468