Can't pull a private repository

## steps

sbt or Giter8 version:

g8 0.16.2

## problem

Trying to run `sbt new` or `g8` on a private repo leads to:

```
> g8 ssh://git@github.com/my-org/my-template.g8.git
ssh://git@github.com/my-org/my-template.g8.git: Cannot log in at github.com:22
```

## expectation

It should not fail to log in (since cloning the repo works just fine)