feat(slugbuilder): build without using a pipe for git archive

Ref #785

This will break builds if the corresponding slugbuilder patch is not merged first.

Blocked by deis/slugbuilder#9. After that is merged tests for this branch will pass.
`git archive` allowed us to specify a branch.  We're not longer doing that.  Though `HEAD` will normally have the code the developer wants to deploy, that may not always be true.  Can we add an explicit checkout of the branch?