Add current branch option to geordi to deploy

[denzelem]

Geordi deploy should have an option to deploy from the current branch / feature branch instead of the pre-configured one (most often this is master).

This should be possible with a command line argument for capistrano.

[codener]

Unfortunately, Capistrano 3 has dropped support for setting config variables from the command line (see https://stackoverflow.com/q/21027452/3497272).

Will set DEPLOY_BRANCH instead, and fail if it isn't used.