Use GCC 4.6+ in CentOS 6 image
In order to update sass-rails
to sassc-rails
we need to bump GCC in CentOS 6 in order for the build to succeed.
This is blocking: https://gitlab.com/gitlab-org/gitlab-ce/issues/18432
sass-rails
is EOL in 26th March 2019, which means we need that before that date.