Update Travis to run Python 3.6
Update Travis to run Python 3.6


Former-commit-id: 34a1711548fa1ba1edc45c2655565f64ddbb3686 [formerly 34a1711548fa1ba1edc45c2655565f64ddbb3686 [formerly 7cd91ae930a3a3a0fd0089e72957311e87a067e0]]
Former-commit-id: c606537dcb1672b4a91797b570b790746123eeed
Former-commit-id: 99d0fcf6d0afb45c15dc4ab8a377fda50a479725
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine