Hack rbenv to make Boxen happy

Because, Boxen installed its Ruby versions in /opt/boxen/rbenv/versions,
but `rbenv init -` insists on adding `~/.rbenv/shims` to the path, rbenv
won't find those. Plus, each time I run the boxen configuration, it will
complain that I have own .rbenv install (because of ~/.rbenv being
created somehow, I blame rbenv invocations for that).