Add failing spec for orders in backend:

This adds a failing spec for orders with both:
- A product assembly
- An item that is a product that also belongs to that assembly
Viewing such orders in the backend causes an sql exception due
to the ambiguos semantics of the assemblies_for method in such
contexts.
Fix variant.assemblies_for query
Advise users the extension is still beta for Spree 2.0
move translation into Spree.t format
setup travis-ci