Users reporting bugs will be able to use these packages to install debug
builds of Background Music without having to install from source. This
is mainly useful because debug builds have more detailed logging.
Hopefully we'll get around to adding an option to enable debug logging
at runtime, but this should work well enough for now.
Also:
- Use newer macOS images in Travis CI builds.
- Fix an xcrun command in build_and_install.sh that was accidentally
being started in the background.
- Fix build_and_install.sh building libPublicUtility.a twice for no
reason.
