It looks like there's a mismatch between the method defined in the commit of Spree you're using and the method in spree_multi_currency.
Spree:
add_to_line_item(variant, quantity, currency, shipment)
Spree_multi_currency:
add_to_line_item(line_item, variant, quantity, currency = nil, shipment = nil)
Make sure you're using the correct branch of the extension to match your Spree branch.