Added a dependency and require for rails_admin/extension.
Using RailsAdmin configured current_user rather than blindly calling
current_user directly.
More fixes for Rails 4.
Further fix, replacing .scoped with .all.

- scoped is deprecated in Rails 4 and removed in Rails 4.1
Merge pull request #1 from undergroundwebdesigns/master

Added a dependency and require for rails_admin/extension.