Require active_support before core_ext to fix uninitialized constant.

This is a known issue with activesupport and travis:
https://github.com/rails/rails/issues/14664

See also:
https://github.com/ManageIQ/polisher/commit/c899d500502a57582a906dfa20cc6cf343c09265
https://github.com/ManageIQ/linux_admin/pull/103