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.
I pulled that down since it forced the user to specify the git repo. If I put it on line 46 then the branch option can only be called by itself. If I try `gem install spree --branch some_branch --git some_repo`, this would fail I believe.

My aim was to keep the `--branch` option global for the user.