Stub for Dir.chdir as it is not needed

spec/tasks/gitlab/backup_rake_spec.rb

@@ -22,6 +22,7 @@ describe 'gitlab:app namespace rake task' do
     context 'gitlab version' do
       before do
         Dir.stub :glob => []
+        Dir.stub :chdir
         File.stub :exists? => true
         Kernel.stub :system => true
       end