This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Try "diskutil eject" twice before "diskutil unmount force".

Use "diskutil unmount force" to unmount DMGs.