Don't use `shell=True` when calling subprocess.Popen.

Using `shell=True` opens the door to argument quoting issues and
little Bobby Tables.  It's unnecessary, let's not do it.

(The salt deploy commands still seem to use magic commas, so if your
repo name contains a comma you will probably have a Bad Day.  But
otherwise we should be pretty robust against strange tag and repo
names now.)
Merge pull request #28 from cscott/quoting

Don't use `shell=True` when calling subprocess.Popen.