I\'ve read and tried the suggestions in several, other, questions, like mine (all with accepted answers) as well as a few more hours of Google searching, but nothing worked.
WOW, JUST WOW.
All I had to do was gem install bundler and then rbenv rehash. Everything worked.
The original error message pointed me to a problem with RBENV or my Ruby version when in reality it was just falling back on an old version of bundler.
Why wouldn't I have received the more standard this needs bundler version [xxxx]... error instead of telling me my Ruby version is specified incorrectly?