Update .rspec

* Remove —warnings.
* Don’t require `spec_helper.rb` as it’s required in `rails_helper.rb`
  which in turn is required in each test file.
Clean up `application` layout
Add `bourbon` sass
Add `neat` sass
Add `bitters` sass
Add `refills` sass
Add `sans` typography from `refills`
Add `progress-bar-indication` from `refills`
Put bourbon goodies in `assets` group