Simplify set_current_order. Fixes #5382

Conflicts:
	core/spec/lib/spree/core/controller_helpers/order_spec.rb
Search for guest order first before running Order.new

Fixes #5441
Add inverse_of for more associations involved in cart processing
Fixes #5488

Conflicts:
	core/app/models/spree/order.rb
	core/app/models/spree/tax_rate.rb
Add circle CI setup
Set specs failing in upstream to pending