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