Thanks for the env info @ctekellogg!
Good to know.
I have one last question --- what do you get when you run the following?
$ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables | grep version
This command should theoretically return something that looks something like this:
version: 9.1.0.0.1.1508540944
although you will probably see a different version number.
After doing a little sleuthing, I think the minimum version of macOS required is 10.10, at least according to this document. Travis CI is involved in the build process for a lot of QIIME 2's third-party dependencies, so this seems like a reasonable target.
If you have a version of XCode < 6.4 on your desktop machine, you might be able to find a 6.4 or greater version on Apple's website, but I can't really say if that is truly an option or not.
Please let us know how it goes! 