Use memory store cache for the Gitlab::Auth

This avoids leaking information into Rails.cache that could influence
other specs.

spec/lib/gitlab/auth_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::Auth do
+describe Gitlab::Auth, :use_clean_rails_memory_store_caching do
   let(:gl_auth) { described_class }
   set(:project) { create(:project) }