Spree add gems with branch

This is in response to issue [#4690](https://github.com/spree/spree/issues/4690).

When installing a rails application, you would be forced to install `2-1-stable` of `spree_auth_devise` and `spree_gateway`. Here, I solve this issue by allowing the user to specify the branch that they would like to use.
That reminds me: congratulations!