Make specs pass.

- Add helper spec.
- Change lambda to expect.
- Set js_errors to false for Poltergeist to make tests pass without
interference from Spree core js issues (you can easy change back to
true).

Fixes #89
Use poltergeist for tests
Bump to Capybara 2.2.0
Add coffee-rails and sass-rails as dependencies so that spree main assets compile