When upgrading from spree 3-0 to 3-1 custom templates stop working

While updating spree version 3-0 to 3-1 the print invoice also got updated apparently since the version did not change. All custom templated stopped working. 

Might be a good idea to do something with the gem version if the complete templating system is changed :/

Forked 3-0-stable and updated gemspec to support spree 3.1.0. Works ;)