Add security warnings to update docs
Bump gitlab-shell in 5.4 and 6.2
Add 5.1-to-5.4 update path
Clarify upgrade steps
Add security update info to 6.0-to-6.2.md
Merge branch 'security_update_master' of /home/git/repositories/gitlab/gitlabhq
Merge branch 'master' of dev.gitlab.org:gitlab/gitlabhq