Switch to xcode10
I suspect this is failing on Mac due to https://github.com/OpenLightingProject/ola/pull/1400/commits/b83e08a71855184ddf027cab4a2298570bee91e5 and the top_builddir probably needs adding to built_sources too.
https://blog.travis-ci.com/2018-07-19-xcode9-4-default-announce
https://blog.travis-ci.com/2018-09-13-xcode-10-is-now-available
Xcode 11 now exists: https://docs.travis-ci.com/user/reference/osx/#macos-version
Tagging as future until we can do free builds on this OS again...