Centos6 is EOL

[jspaleta]

Looks like centos6 public mirrors for packages are dead.
http://mirror.centos.org/centos-6/6/readme

Docker base images may be available, but you can't install packages on them without a ton of effort to find a package repository that is still active.

We may still want an el/6 rpm target for a couple of reasons.

1. Amazon Linux reports to be most compatible with el/6 and package cloud currently maps amzn linux to el/6
2. customers may have RHEL 6 extended support

Impact:
This is mostly gonna impact things like asset building... ruby runtime asset from sensu-community org for example can't build against centos6 any longer.