Upgrade to ActiveAdmin 0.6.0 (Fixes /dashboard link)

Migrates from old ActiveAdmin dashboards.rb to new pages model with default admin homepage of dashboard.rb.

Dashboards were [deprecated in ActiveAdmin 0.5.0](http://reverbhq.com/blog/2012/08/activeadmin-0-5-0-is-released/) and [removed in ActiveAdmin 0.6.0](https://github.com/gregbell/active_admin/blob/master/CHANGELOG.md#060)