Improve `inventory_units` on OrderIventoryAssembly

It was querying line items when there was no need to
Attempt to improve the extension use case description
Run build on spree/spree master and 2-0-stable
Add call to sort_packages and method placeholder
Check parts stock in Stock::AvailabilityValidator

Since https://github.com/spree/spree/commit/fcb4293046208a05dd9b281b1d767d588550b904
stock is no longer checked within OrderPopulator so we need to check for
parts stock on line item validation which uses
Stock::AvailabilityValidator in turn
Bump rspec-rails 2.14.0