website: Added missing python-six dependency for Ubuntu 18.04

Change-Id: I32e0a5606149568dcf592fbe2200262ed413c47e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/28871
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed