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

Some specs fail in rails5 branch with errors like

```
1) Group#secret_variables_for when the ref is not protected contains only the secret variables
    Failure/Error: variables = Ci::GroupVariable.where(group: list_of_ids)

    NoMethodError:
      undefined method `arel_table' for Ci::Group:Class
```

This commit fixes it.

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