Xcode version gets reported in travis ui even for linux builds #5125


[Sorta spun out of issue 3975.]

If a build is mult-ios, it seems the Xcode version gets reported even for platforms where the info doesn’t apply.

Google’s protobuf setup builds on linux and OS X. Since issue 3975 recently got fixed, we were able to add in Objective C testing to the mix of languages by adding the osx_image tag and extending out script. But now it seems every build, even the ones on linux, list the Xcode version. Example: https://travis-ci.org/google/protobuf/builds/91337154

This info should only get reported for OS X builds where it applies.