Require Spree 2.1.0.beta
Remove references to methods removed in spree/spree

So that we can actually boot the app
Spree::Variant no longer has a `on_hand` attribute

Instead stock it should be check through variant stock items. This makes
it possible to go through checkout without raising errors
Allow test to run

Update factory girls and test helpers path
Remove specs that give no value to the extension

They were basically testing spree core functionalty. No need to maintain
those here