Reproduce case where promo doesn't apply

If promo is not eligible at the time the line item was created and
becomes eligible for some reason when order is updated later on the line
item discount is not applied
Introduce a PromotionPool to manage promo adjustments

So far makes it possible to grab all eligible actions and properly
update their adjustments amount and eligibility
Remove OrderPopulator overrides

As the extension no longer needs its promo action to run through
`promotion.activate`, neither it needs promo event names or AS
Notifications
Decorate `update_adjustments` to run line item discounts
Make PromotionPool also invalidate adjustments
No need to care about eligibility when creating the adjustment
Add .travis.yml