add counter cache from variants to stock items

fixes #5520
check inventory and restart checkout if not enough

fixes #5585
Update ruby version for circle builds
fixes #5585
Scope to orders line items. Fixes #5594
Duplicate entry of inventory_unit_attributes in ATTRIBUTES, its already present at line number 18
Fix Asset position for different viewables.
fixes #5590
Update MoveOrderTokenFromTokenizedPermission migration to work w/ jruby

jruby mysql adapter uses adapter_name 'MySQL'. Not recognizing this causes migrations to bomb out on jruby due to invalid SQL syntax.

https://github.com/jruby/activerecord-jdbc-adapter/blob/master/lib/arjdbc/mysql/adapter.rb#L109

fixes #5597
Validate authenticity token on all actions but populate

fixes #5601

Conflicts:
	frontend/app/controllers/spree/orders_controller.rb
Add total on hand to variant RABL

added test coverage for new rabl fields

fixes #5593

Conflicts:
	api/app/views/spree/api/variants/small.v1.rabl
Skipping the set_current_order before filter on the cart link action as this should use the simple_current_order helper only

fixes #5605
Don't load the store if we don't have to - minor optimisation but every bit helps
fixes #5606
Add is_backorderable to variant and small RABL view
fixes #5595

Conflicts:
	api/app/views/spree/api/variants/small.v1.rabl
	core/app/models/spree/variant.rb
Fix git conflict.
Fix product specs.
fix use billing
Test latest 2.1.5

Conflicts:
	.travis.yml

Conflicts:
	.travis.yml
Fixes #4905: Error while trying to set "weight" in Variant with "," as seperator.

Fixes #4905
Fixes #4939
Refactor number parsing to be DRY. Fixes #4939

Conflicts:
	core/lib/spree/core.rb
	core/spec/models/spree/variant_spec.rb
	core/spec/spec_helper.rb
Fixes product validation on create. Fixes #5588

Conflicts:
	api/app/controllers/spree/api/base_controller.rb
	core/app/models/spree/product.rb
	core/app/models/spree/variant.rb
Fix git conflict.