Updated assets structure for changes in Spree 2.2

See change details at:
https://github.com/spree/spree/blob/2-2-stable/core/CHANGELOG.md#asset-renaming

Conflicts:
	app/assets/javascripts/spree/backend/spree_digital.js
	app/assets/javascripts/spree/frontend/spree_digital.js
	lib/generators/spree_digital/install/install_generator.rb