Fallback to IPv4 pool to fix keyserver receive failed in Travis CI builds

[rbreslow]

Overview

Travis CI builds were failing, seemingly at random in azavea/docker-django#56. After further investigation, it turned out this was happening because IPv6 networking for Docker is disabled by default, and the SKS Keyservers high-availability pool contains some IPv6-only servers:

• gpg: keyserver receive failed: Address not available nodejs/docker-node#380 (comment)
• IPv6 sockets returning "Cannot assign requested address" travis-ci/travis-ci#8711 (closed due to inactivity)
• https://sks-keyservers.net/status/

While working on docker-django, I referenced the Travis CI builds for this repository, and saw that many of them were failing as well.

This PR adds a conditional expression to fallback to the IPv4-only SKS pool if we get an IPv6 server from the high-availability pool.

Testing

See Travis CI build: https://travis-ci.com/rbreslow/postgres/builds/78322372

https://travis-ci.com/rbreslow/postgres/jobs/133223806:

+ gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4
gpg: keybox '/tmp/tmp.xo4ImMRNSu/pubring.kbx' created
gpg: keyserver receive failed: Cannot assign requested address
+ gpg --keyserver ipv4.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4
gpg: /tmp/tmp.xo4ImMRNSu/trustdb.gpg: trustdb created
gpg: key 036A9C25BF357DD4: public key "Tianon Gravi <tianon@tianon.xyz>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
+ gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu
gpg: Signature made Wed Sep 28 22:22:56 2016 UTC
gpg:                using RSA key 036A9C25BF357DD4
gpg: Good signature from "Tianon Gravi <tianon@tianon.xyz>" [unknown]
gpg:                 aka "Tianon Gravi <tianon@debian.org>" [unknown]
gpg:                 aka "Tianon Gravi <tianon@dockerproject.org>" [unknown]
gpg:                 aka "Andrew Page (tianon) <andrew@infosiftr.com>" [unknown]
gpg:                 aka "Andrew Page (tianon) <andrew@vitalroute.com>" [unknown]
gpg:                 aka "Andrew Page (Tianon Gravi) <admwiggin@gmail.com>" [unknown]
gpg:                 aka "Tianon Gravi (Andrew Page) <tianon@infosiftr.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: B42F 6819 007F 00F8 8E36  4FD4 036A 9C25 BF35 7DD4

[tianon]

docker-library/php#666 has been really successful over in PHP, so I'd much rather take that approach (especially since that's exactly what all the official build servers do now also).

[tianon]

#471 👍