Ruby 2.1 master broken in EE

See: https://gitlab.com/gitlab-org/gitlab-ee/pipelines/5961441/builds

For example: https://gitlab.com/gitlab-org/gitlab-ee/builds/9081340

rake aborted!
NoMethodError: super: no superclass method `included' for EE::Namespace:Module
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/concern.rb:130:in `included'
/builds/gitlab-org/gitlab-ee/app/models/namespace.rb:4:in `prepend'
/builds/gitlab-org/gitlab-ee/app/models/namespace.rb:4:in `<class:Namespace>'
/builds/gitlab-org/gitlab-ee/app/models/namespace.rb:1:in `<top (required)>'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:457:in `load'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:457:in `block in load_file'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:647:in `new_constants_in'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:456:in `load_file'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:354:in `require_or_load'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:494:in `load_missing_constant'
/builds/gitlab-org/gitlab-ee/vendor/ruby/2.1.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:184:in `const_missing'
/builds/gitlab-org/gitlab-ee/app/models/group.rb:5:in `<top (required)>'

/cc: @rymai