Simplify Ubuntu versions used for Gitlab CI to avoid need for future updates

Instead of:
- 14.04 (LTS release)
- 16.04 (LTS release)
- 17.10 (non-LTS release)

Use:
- latest (latest LTS release)
- rolling (latest release - could be LTS)
- devel (whatever the devel suite points to)

See https://store.docker.com/images/ubuntu for more information