starbase_archive cannot extract non-tarred .gz files

[Phault]

In the case of pact_verifier_cli, they upload gzipped binaries, but since it's a single file they do not tar it first.

It appears starbase_archive currently assumes that any file with a .gz extension must be a .tar.gz file, so a lovely error is spit out:

numeric field was not a number:  when getting cksum for ����