Advise users the extension is still beta for Spree 2.0
move translation into Spree.t format
setup travis-ci
Account for situation where we can't find a unit line item

Users upgrading from older versions could see exceptions because the
extension may not be able to find which line item originated a inventory
unit
Add task to help users link units to line items

So that they have a chance to look at the backend UI and tell which
items belongs to which product bundle
Merge remote-tracking branch 'upstream/bundle-product' into assembly_variants
merged latest from original repo's bundle-product and master branches