kit sync actually updates the kit now.

Was updating the stock item that belonged to a variant with the id of
the product being passed in ;_;

Change-Id: Iccd8fcd5e12ae8239d67e465239f63e03f42d513
fix nil stock item issue

issue occurred when trying to update the stock of an assembly and the
stock item didn't exist.

Change-Id: I3bc60bfd7d3b3526e896f6fa6595d581fd4cbed9
Merge pull request #8 from Dkendal/update_assembly_inventory

Update assembly inventory