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.
installer command accepts -applyChoiceChangesXML option to change
customize options on the GUI installer from the commandline.
(`man installer` for more detailed information)
The introduced option `choice` enables the choice changes to be supplied
via pkg stanza without tricks in preflight code.
