update README with the latest version of stable engine
update README with the latest version of stable engine


Former-commit-id: fad3f0456a6e126ccf7145222219e702904762fc [formerly 0610786e0a8dba5cc3307f2ea28bcc77666c8e3a]
Former-commit-id: 40b68ecc261c759abda1410b0c955cc99cd557af
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine