Add a check to prevent skipping checkout steps

fixes spree/spree#2280
Change in amount precision limit

Fixes #2715
Fixes #2716
Change in amount precision for spree adjustments

Fixes #2715
Fixes #2716
Update extension generator dependency versions.

Fixes #2743
Empty user promotion rule should not be eligible

Fixes #2749
Use capybara_ext provided by spree core.

Fixes #2742
Run extension installation generator during rake test_app.

Fixes #2741
Minor refactor: extract gateway order id to method

This will allow people to more easily override the order id passed
in to the payment gateway.  It will behave the same for existing
functionality.

Fixes #2732
Correct filename for diners club credit card icon

Fixes #2726
Remove separate rails components from gemspec as we are requiring rails itself.

Fixes #2725
Fix styles for empty orders list in admin

Fixes #2734
Fix email validation error I18n symbol lookup

And include %{value} for interpolation in validation messages.

The current code hardcodes the I18n key and improperly assigns the error
which leads to several problems:
* You cannot override the translation message for models/attributes
* You cannot pass your own I18n key as a symbol (eg. :invalid_email)
* You cannot use symbol interpolation

This fixes the above problems and is the proper solution for issue #2346.

This fix also applies to 1-2-stable and 1-3-stable.
For a fix against 1-1-stable see PR #2729.

Fixes #2730
Add simplecov for spec coverage reporting.

Fixes #2723
Modify the look of the coupon code.

This moves coupon code to be better aligned with the cart buttons.
This significantly improves the way things look in alternate
localizations.

Fixes #2720
Improve documentation about using gem from GitHub
Improve documentation about using gem from GitHub

Fixes #2705
Tidy up ProductDuplicator class, add tests + fix #2684
Correct content length check in base_helper_spec test for meta_data_tags