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.
Ah, good point. It's my first day back at work today after three weeks so I'm still a bit rusty.