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.
The slugbuilder user can't access this directory (it was created by the user root and `mkdtemp` only gives access to this user)
(just change the permissions)
```
temp_dir = tempfile.mkdtemp(dir=build_dir)
os.chmod(temp_dir, 0755) 
```