Remove Order payments association's inverse_of

This was causing spree/spree_gateway#132, and would've caused similar issues for all non-token gateways (i.e. gateways that send CC info rather than getting a token and using that instead)

Super incredibly glad that we found this out before master was released as a real gem.

core/spec/models/spree/order_spec.rb

@@ -47,6 +47,7 @@ def compute(computable)
   end
 
   context "#payments" do
+    # For the reason of this test, please see spree/spree_gateway#132
     it "does not have inverse_of defined" do
       expect(Spree::Order.reflections[:payments].options[:inverse_of]).to be_nil
     end