1.3.1 [Bugfix] - Node 6 compatibility

Things we fixed

• The new template tests we introduced in 1.3.0 used the object rest spread operator, which is only available in node >=8. Since the template should work with node >=6, we fixed this and switched to a node6 docker image on circeci to ensure this won't happen again. (8ce0da4 & e1ba1ea, closing #1245)