ActiveAdmin support for Rails 4/4.1 requires github specification

Plain gem 'activeadmin' was failing under Rails 4/4.1 and https://github.com/gregbell/active_admin doc indicates it is still proper to use:

```
gem 'activeadmin', github: 'gregbell/active_admin'
```

I've added that to the recipe and specs and local gem testing now works when trying to build sample apps with ActiveAdmin.