Stop building packages for Ubuntu 14.04
Ubuntu 14.04 reaches EOL on April 2019 - https://wiki.ubuntu.com/Releases. This means, GitLab 11.10 would be the last version with packages for Ubuntu 14.04.
- Announce in 11.10 release post that it will be the last release for Ubuntu 14.04
- Once 11.10 is out, remove Ubuntu 14.04 jobs from CI