Use rbenv to change Ruby versions

* Use Homebrew consistently for OS programs.
* Use ruby-build to install Rubies.
* Include auto-switching of Ruby in `~/.zlogin`.
* Upgrade Rubygems after Ruby is installed. This is to help avoid common
  Bundler/Psych issues:
  https://github.com/tenderlove/psych/issues/66
* Use newest Bundler version in order to work with latest Rubygems.
* Use rbenv-gem-rehash so the shell automatically picks up binaries
  after installing gems with binaries.