If kerberos is enabled require it

Since 'group: :kerberos' got [included in the Gemfile ](https://gitlab.com/gitlab-org/gitlab-ce/commit/2391a43a7073b4e723e9ca1f7ff19b30f8e51452) Kerberos no longer got [loaded when the app started](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/application.rb#L6)

This caused `Devise::OmniAuth::StrategyNotFound` error once Kerberos is enabled, failing to start GitLab .

With this we hopefully do not need to remove group from the Gemfile.

See merge request !819