Roll tarballs slightly differently so we actually get compression.

We were relying on "git archive" being able to create tar.gz files,
but that ability isn't present in the git that comes with CentOS 6,
so we use git to create a tar file and then compress it ourselves.
(cherry picked from commit 49a345cc023092a5503e7bb965742f8f1a7b5938)

Signed-off-by: Bruce A. Mah <bmah@es.net>