Delegate Spree.t to Rails TranslationHelper

This way we get all the nice features Rails t() helper offers, mainly
displaying reasonable strings for missing translations.

At this point core test suite should be all green. I've remove a couple
i18n specs which were translating to :en locale. I don't think that make
sense since :en is the default locale. Other translations specs pass now

[Fixed #3023]
Fix scss import.

[Fixes #2932]
Put variables back where it was.

[Fixes #2422]
add the template file for generating the .rspec file. this was causing
"Could not find "rspec" in any of your source paths. Your current source
paths are: " [Fixes #3031]
Add request spec for backend new order process [Fixes #3022]

Fix order details and shipping method request specs

[Fixes #3000]
Bump caypbara to 2.1.0:

* Add timeout into config/database.yml template to avoid SQLite3 DB locked out
* Use lenient non-exact setting for Capybara element matcher (same as 1.x)
Capybara 2 updates.