Fix compatibility with latest Spree stable release

	Gemfile
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
(cherry picked from commit ebcf0e8501d704ccbbba20539ed5c05bcc39b724)

Signed-off-by: Tobias Bohwalli <hi@futhr.io>