Python 2.7.13.

[rillian]

Also move to the tar.xz package, which is a 27% smaller.

[rust-highfive]

r? @brson

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[rillian]

Also note that if you haven't rebuilt the builtbot containers recently, please check that you've got the fix for https://rhn.redhat.com/errata/RHSA-2016-2962.html. I had to do docker build --no-cache=true to re-run the dnf update step; the docker.io/centos:5 image doesn't appear to have it.

[alexcrichton]

Thanks!