Use updated ruby in our builders builder
This is the corresponding update for https://gitlab.com/gitlab-org/gitlab-omnibus-builder/merge_requests/57
That change also ended up switching our debian build image from Debian 9, to Debian 9.1
There is an issue with building native extensions if you are using a ruby built on the wrong version, so we need to introduce some change to bust the debian 9 ruby cache in omnibus. (In order to make sure it rebuilds ruby)
In this MR I busted the cache by changing our CI job name to Debian 9.1, there may be a nicer way to do it. Open for suggestions.