[test] Use `xcode8.1` image to test macOS code
[test] Use `xcode8.1` image to test macOS code

Ref: https://blog.travis-ci.com/2016-11-15-xcode-81-is-here

christophermoura/dotfiles,girishramnani/dotfiles,pbrooks/dotfiles,girishramnani/dotfiles,christophermoura/dotfiles,AlbertoElias/dotfiles,mickael83/ubuntu-dotfiles,makabde/dotfiles,b-boogaard/dotfiles,makabde/dotfiles,mickael83/ubuntu-dotfiles,AlbertoElias/dotfiles,oddlord/dotfiles