Update Heroku-20 installed packages list (#170)
Update Heroku-20 installed packages list (#170)

The Travis cron on master is currently failing since the generated
`heroku-20/installed-packages.txt` file doesn't match the checked in
version:
https://travis-ci.com/github/heroku/stack-images/jobs/362055551

This is because on 2020-07-16 a new version of `librsvg2-2` was released
for Focal, which added `libcairo-gobject2` to its `Depends` list:
https://ubuntuupdates.org/package/core/focal/main/updates/librsvg2-2
https://packages.ubuntu.com/focal-updates/librsvg2-2

Packages being added (vs removed) is non-breaking, so no changes are
required beyond updating the installed packages list to reflect reality.

Fixes W-7836044.
heroku/stack-images