Merge branch 'blackst0ne-rails5-fix-ci-group-class-arel-table-error' into 'master'

[Rails5] Fix `undefined method 'arel_table' for Ci::Group:Class` error

See merge request gitlab-org/gitlab-ce!18329

app/models/ci/group_variable.rb

@@ -4,7 +4,7 @@ module Ci
     include HasVariable
     include Presentable
 
-    belongs_to :group
+    belongs_to :group, class_name: "::Group"
 
     alias_attribute :secret_value, :value