• Added an rbenv root command which prints the value of
  $RBENV_ROOT, or the default root directory if it's unset.
• Clarified Zsh installation instructions in the Readme.
• Removed some redundant code in rbenv rehash.
• Fixed an issue with calling readlink for paths with spaces.
• Changed Zsh initialization code to install completion hooks only for
  interactive shells.
• Added preliminary support for ksh.
• rbenv rehash creates or removes shims only when necessary instead
  of removing and re-creating all shims on each invocation.
• Fixed that RBENV_DIR, when specified, would be incorrectly
  expanded to its parent directory.
• Removed the deprecated set-default and set-local commands.
• Added a --no-rehash option to rbenv init for skipping the
  automatic rehash when opening a new shell.