Update location of download tarball

The tarball is programmatically downloaded and removed; if for some
reason it fails to remove, the script won't exit (as intended) and
the tarball is left.  Moving the tarball to the /tmp directory is
where traditionally tarballs[1][invisibles] are temporarily stored so
that when the computer restarts, the /tmp folder will be emptied[2][tmp]

[tmp]: http://superuser.com/a/581732/298006
[invisibles]: http://www.westwind.com/reference/OS-X/invisibles.html

Issue: #3