Bump to allow any highline version in the > 1.6.18 & < 1.7.x range

Fixes #3870
Fix issue where Product#set_property was causing an undefined method with spree_i18n

Fixes spree/spree_i18n#301

Conflicts:
	core/app/models/spree/product.rb
Select2 fixes to center the search box

Fixes #3854
Fixes #3863
Add process method to BogusSimple gateway to be able to process payments.

Fixes issue where hitting "process" on payments screen breaks with
BogusSimple gateway.

Fixes #3824
Fix issue where Product#set_property would sometimes complain about a missing property association
checkout/registration view should live in auth component, not frontend
Bump activemerchant to 1.40.0

Likely fix for spree/spree_gateway#105
Fix sporadically failing new_order_spec
Split creating payments + void test into two separate tests
use correct casing in payments_spec for 'paid' check