Use roadie-rails

roadie 3.0.0 is released! and it works with roadie-rails on Rails.
Enable roadie in automatic
Apply stylesheet

roadie 3.0 doesn't have `css` option.

The way to add link to stylesheet is recommended:
* https://github.com/Mange/roadie-rails/blob/master/Upgrading.md#mailers
Merge pull request #37 from tricknotes/roadie-3

Upgrade Roadie 3