Bionic breaks package signing because importing the private key is unsuccessful. Works in xenial. The buildbot distro doesn't matter because we're just sending source packages to Launchpad which builds on the target distro.

This reverts commit 8780800e.

(it was never in the original travis script)

* Use sdl2_mixer from mazmazz/srb2 due to removed options from homebrew-core
* Use `brew install --build-bottle` to build compatible libraries for old Macs

This reverts commit fd12e856.