update macOS builder to 10.13

[Be-ing]

The macOS builder is still running macOS 10.11. Eventually this will prevent us from upgrading the macOS SDK to the version required by Qt.

[Be-ing]

Shall we target this for 2.3?

[daschuer]

I think we can close this one.
It will be pop up again when 10.11 falls out of support from Apple.

[Be-ing]

Let's not wait for things to become problems before fixing them.

[rryan]

This was easier than I thought it would be. The VM is ready (in Jenkins as build-macosx-10.13-amd64) but I need @Pegasus-RPG to accept the host SSH key from the Jenkins machine.

I will update https://github.com/mixxxdj/buildserver/blob/2.2.x-unix/scripts/macosx/SETUP.md with the latest instructions for settting up a macOS builder.

[Be-ing]

@rryan remember to update the Jenkins configuration scripts to point to the new version of the macOS SDK (#59).

[rryan]

I already updated the Jenkins config to detect the installed SDK instead of hard-coding a version. :)

[rryan]

I also have a PR that I'll open once the new VM is enabled that updates build_environment.sh to auto-detect the SDK by default.