Update the ubuntu images we use in tor-browser-build
pospeselr told me that the ubuntu 17.04 images are missing under http://cdimage.ubuntu.com/ubuntu-base/releases/.
We should switch to the images from 17.10, or maybe 16.04.3 (LTS). We only use this image for debootstrap, so it does not matter which image we use, as long as its debootstrap is not too old.