Clear tax adjustments only in TaxRate#adjust

It doesn't make sense to remove all line item adjustments from
TaxRate#adjust because price adjustments might come from other classes
such as ShippingMethod and PromotionAction

Fixes #2981

Conflicts:
	core/app/models/spree/order.rb
Fix order deprecation on association.

Conflicts:
	core/app/models/spree/order.rb
Fix deprecation warnings by using line_item_adjustment_totals.
Use order option not a scope for Rails 3.2.x.
Add sample gem to travis config.