work around transient (?) problem with apt-get update

[SethTisue]

so we can get this 2.13.9 show on the road

[SethTisue]

I guess I'll revert this post-release, but I should add a comment referencing this PR, in case it happens again

failure details were

W: The repository 'http://apt.postgresql.org/pub/repos/apt xenial-pgdg Release' does not have a Release file.
217E: Failed to fetch http://apt.postgresql.org/pub/repos/apt/dists/xenial-pgdg/main/binary-amd64/Packages  404  Not Found [IP: 87.238.57.227 80]
218E: Some index files failed to download. They have been ignored, or old ones used instead.

[SethTisue]

haven't forgotten about this, but now waiting until after 2.13.10 is out

[SethTisue]

reverted, post-2.13.10, in 13f3ea4 — but perhaps that isn't the end of it