WIP Install crossfader command as a standard gem

Without this patch it is way too complex to manage the actual
installation of the `crossfader` command and dependencies into the
runtime ruby that's specifically intended to manage the rest of the
available Ruby versions.

This is a problem because the user facing entry point and user facing
layout of the `crossfader` tool is too complicated to quickly dive in
and improve.

This patch addresses the problem by removing all notions of gemsets from
the core runtime.  The default system gem directory will be used and
dependencies will be managed simply by performing a gem install once the
ruby runtime has been built.

It's important to note this design and layout only apply to the Ruby
runtime for crossfader itself.  The ruby versions that are activated by
the crossfader tool are still independent and will have unpolluted
system gem repositories.