Merge pull request #741 from hflynn/rebased/develop/remove_unsupported_ubuntu

Remove Ubuntu versions which aren't supported by upstream (rebased onto develop)
merge develop -D org

Repository: openmicroscopy/ome-documentation
Excluded PRs:
  # PR 744 rleigh-dundee 'Use ant to build sphinx docs' (status: failure)
  # PR 743 sbesson 'Propose redesign of OMERO and Bio-Formats 5.0 CI jobs' (exclude comment)
Already up-to-date.

Generated by OMERO-5.1-merge-push#162 (http://ci.openmicroscopy.org/job/OMERO-5.1-merge-push/162/)