update changelog for 1.8.0 release (engine-1.8)
update changelog for 1.8.0 release (engine-1.8)


Former-commit-id: 24a6ded0f7f22ee0938ab473334a7abf3cd8c038
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine