Use described_class

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8088#note_20133505

spec/lib/gitlab/serialize/yaml_variables_spec.rb

@@ -2,8 +2,7 @@ require 'spec_helper'
 describe Gitlab::Serialize::YamlVariables do
   subject do
-    Gitlab::Serialize::YamlVariables.load(
-      Gitlab::Serialize::YamlVariables.dump(object))
+    described_class.load(described_class.dump(object))
   end
   let(:object) do