Change the table of supported Ice/OMERO versions.

Note Debian 6.0 supports ends in May 2014 and provided ice-3.3. Restricting the supported versions to 5.0 instad of 5.x gives us the opportunity to drop ice-3.3 if we wish.
Reformat table
Merge pull request #710 from manics/linux_versions

Update supported Linux versions for 5.0