use, if defined, GEM_MASTER_BRANCH env variable value instead 'master' string
use, if defined, GEM_MASTER_BRANCH env variable value instead 'master' string


Former-commit-id: a8dc5ac23b53a694dfbfa42a27c41a2b8593a016
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine