Fix Travis conf
- make sure `.env` file is available
- remove IRC notifications
- remove unnecessary config options
https://trello.com/c/5L59pIZJ
Pull out RailsAdmin conf from models classes to an initializer
Add Devise based authentication for RailsAdmin
* the model User has a boolean `admin` field
* add God admin account in seeds
* users can't sign up by themselves
* cleaned up devise views markup and content (in french)
https://trello.com/c/FgCacpo9