Spree::ActiveShipping::Base#compute should only ever take a Spree::Stock::Package object

Fixes #96
Make 'spree' line in Gemfile friendly to bundle.local configs
Make active_shipping dependency friendly to being used as a bundle config local. dependency
Begin putting in place a real testing framework for this extension
Replace the stock_location_id instance variable.

Fixes #98
Only run 1.9.3 and don't spam IRC
Remove @stock_location variable after discussion in #98