Lazyload variant associations in api/variants
Remove useless return in the show action from the products controller

Fixes #4635
Update CONTRIBUTING.md

Remove versions of Spree and Rails, as that info is provided by the Gemfile.
Update CONTRIBUTING.md

Require gemfile.lock too.
More reasonable order payment state logic

For simplicity sake. Admin users can still go to the orders specific
payments section for more detailed info on payments