GitHub download failed on 13:50:36
https://ci.appveyor.com/project/ruby/ruby/build/1.0.9221 and it also
failed on 13:51:35 (all builds between them failed too).
It means that we need to expect GitHub 502 that continues 1 minute.
So I configured 6 retries, that will sleep at most 91s in total.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64317 b2dd03c8-39d4-4d8f-98ff-823fe69b080e