Fallback to IPv4 pool to fix keyserver receive failed

[rbreslow]

Overview

Travis CI builds were failing, seemingly at random. 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/

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.

Fixes #56

Testing

See Travis CI build: https://travis-ci.org/azavea/docker-django/jobs/400941173

---> Running in 01cb1a99d43e
+ key=B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
+ mktemp -d
+ export GNUPGHOME=/tmp/tmp.FErDewuJ8q
+ gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
gpg: keybox '/tmp/tmp.FErDewuJ8q/pubring.kbx' created
gpg: keyserver receive failed: Cannot assign requested address
+ gpg --keyserver ipv4.pool.sks-keyservers.net --recv-keys B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
gpg: /tmp/tmp.FErDewuJ8q/trustdb.gpg: trustdb created
gpg: key 7FCC7D46ACCC4CF8: public key "PostgreSQL Debian Repository" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1

[hectcastro]

Nice sleuthing. 👍

[rbreslow]

Interesting that this hadn't come up beforehand!