Require gitlab-shell 1.6.0

lib/tasks/gitlab/check.rake

@@ -657,7 +657,7 @@ namespace :gitlab do
   end
   def check_gitlab_shell
-    required_version = Gitlab::VersionInfo.new(1, 4, 0)
+    required_version = Gitlab::VersionInfo.new(1, 6, 0)
     current_version = Gitlab::VersionInfo.parse(gitlab_shell_version)
     print "GitLab Shell version >= #{required_version} ? ... "