deb: make the postinst working on usupported distros
deb: make the postinst working on usupported distros

Former-commit-id: c98022e891cec5c1d232a3146cd2b345bc604965 [formerly 4c1d26d8606539ae19c76c7109791af66d5b133a]
Former-commit-id: 12bd9f5ffe7afd9e201629e3c202842368afa9b6
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine