My problem was that I did:
sudo gem install bundler
So I had installed as root rather than as myself. So I uninstalled as root, then installed as myself:
sudo gem uninstall bundler
gem install bundler
rbenv rehash
(last command for if you are using rbenv)
And it worked. The “correct” path was in .bashrc (or other shell profile), at least according to
$PATH
=> zsh: /Users/myself/.rbenv/shims:/Users/myself/.rbenv/bin: ... etc
but it was expecting it to be installed for myself – not for root. In my case, its rightful installation place is in ~/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/bundler