travis: use default osx_image
travis: use default osx_image

This upgrades to xcode9.4 (current default) and that adds support for C++17,
as required since (upcoming) Uno version 2.0.0-beta.3.

fusetools/fuselibs-public,fusetools/fuselibs-public,fusetools/fuselibs-public,fusetools/fuselibs-public,fusetools/fuselibs-public,fusetools/fuselibs-public,fusetools/fuselibs-public