Adding cmdRunner field to Copy(). Fixes #4

A previous [commit](https://github.com/progrium/configurator/commit/8777f79c8d7d3e85106823ace120063d95011b5e#diff-b4bda758a2aef091432646c354b4dc59R46) introduced the `cmdRunner` field but the `Copy()` method missed it.

## Testing this pull-req

With [Dockerfile](https://gist.github.com/lalyos/075ff4b62c9ae5145109#file-dockerfile) referenced from Issue #4 you can test any repo/branch:

```
docker run --rm \
  -e REPO=github.com/lalyos/configurator \
  -e BRANCH=nil-pointer-dereference \
  configurator-test
```