Use VCS source to fix REVISION errors

Gitlab gets its own revision from the git history. If the gitlab
directory is not a git repository it shows its own version like this:
"GitLab 7.4.2 fatal: Not a git repository (or any of the parent
directories): .git"

Found in the gitlab source:

$ ag REVISION
config/initializers/2_app.rb
3:  REVISION = Gitlab::Popen.popen(%W(git log --pretty=format:%h -n1)).first.chomp