bundle install fails for master on centos 6

Summary

With the introduction of sassc-rails, in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8556, bundle install now fails on centos 6 due to centos 6 having an old version of gcc

see: https://github.com/sass/sassc-ruby/issues/37

Relevant logs and/or screenshots

Example failing build from omnibus: https://dev.gitlab.org/gitlab/omnibus-gitlab/builds/255975