End of life for CentOS 6

We should stop building packages for CentOS 6 soon because its End-of-Life date is November 30, 2020 (https://wiki.centos.org/Download).

CentOS 6 only has gcc v4.4, but many packages now require -std=c++11, which requires gcc 4.7+, to build:

1. libsassc: gitlab!27267 (merged)
2. charlock_holmes: gitaly!1694 (closed)
3. google-protobuf: gitlab!27741 (merged)

I don't know how many customers still use CentOS 6, but I imagine there are a handful of them out there.