Use xcode7.3 for macOS builds
Use xcode7.3 for macOS builds

This fixes deployment issues resulting in missing macOS binaries [1]. `xcode7.3` is currently the default on TravisCI [2].

[1] http://forum.world.st/Bleeding-edge-build-for-Mac-missing-td4981167.html
[2] https://docs.travis-ci.com/user/reference/osx/#OS-X-Version

/cc @krono
OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm