Fix specs

This PR includes a couple of changes to fix specs:

1. Remove attr_accessible, because we depend on Spree::Core, which depends on Rails ~> 4.0.0. 
2. Gemfile is configured against master of active_shipping, which has bumped up a minor version.  So update the gemspec to allow 0.10.x versions.
3. Fix a bad argument in a spec (an Order was being passed to a Calculator when a Package was expected), and use FactoryGirl for the associated objects to get the spec green
4. Added Ruby 2.0 to the Travis CI file.

There's a blocking issue on spree_core - https://github.com/spree/spree/pull/3584 - which prevents specs from running clean on CI, but once that issue is addressed and this is merged in the specs should run green.