Add a test to ensure this works on MySQL

spec/migrations/update_retried_for_ci_builds_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20170503004427_upate_retried_for_ci_build.rb')
+
+describe UpateRetriedForCiBuild, truncate: true do
+  let(:pipeline) { create(:ci_pipeline) }
+  let!(:build_old) { create(:ci_build, pipeline: pipeline, name: 'test') }
+  let!(:build_new) { create(:ci_build, pipeline: pipeline, name: 'test') }
+
+  before do
+    described_class.new.up
+  end
+
+  it 'updates ci_builds.is_retried' do
+    expect(build_old.reload).to be_retried
+    expect(build_new.reload).not_to be_retried
+  end
+end