Now Ruby comes with its own bundler let's favour using it when we can
over requiring a system one be installed.
This avoids needing to have anything in `~/.gem` again.
I am somewhat optimistic this may help with #6579 but it's useful
by itself.
Rather than trying to be smart and doing this ourselves in `brew cleanup` let’s just installed Bundler somewhere it doesn’t try to clean itself up and use `bundle install --cleanup` when we need cleanup done.
Also, use `ohai` and `odie` when possible as they look nicer.
