Add .rubocop.yml

This is the default Hound ruby style config taken from:
https://github.com/thoughtbot/hound/blob/1f03cb64743765801a81c0f7ca9df64a448f1d48/config/style_guides/ruby.yml

These are the checks that Hound currently runs on every pull request.
This config allows people to run rubocop on their local boxes to see
what Hound is going to complain about when they push code.

Closes #5649
add rubocop to the common spree test and development dependencies.
add bullet to the sandbox Gemfile
Allow returned orders to transition to returned

This will allow orders where the entire order was exchanged and then entirely returned to be processed correctly.