travis: Use xcode9.4
travis: Use xcode9.4

https://blog.travis-ci.com/2018-07-19-xcode9-4-default-announce
dreal/dreal4,dreal/dreal4,soonho-tri/dreal4,soonho-tri/dreal4,soonho-tri/dreal4,dreal/dreal4,dreal/dreal4,soonho-tri/dreal4