fix(builder): disallow password authentication

Ref: #1340

Replaces the previous password prompt with an error message similar to:

    kamina ~/tmp/deis/example-go (git:master) ✔
    ❯❯❯ git push deis master
    Permission denied (publickey).
    fatal: Could not read from remote repository.

    Please make sure you have the correct access rights
    and the repository exists.

Ideally we would brand this with a Deis-centric message, but for now
this should be good enough until we overhaul the builder completely.