rbenv-update fails without RBENV_ROOT

- RBENV_ROOT is not set by any of the default rbenv configuration
  options, only the shared configuration. This means that rbenv-update
  does the same as 'cd ~' without this being set.
- rbenv-root provides the value that should be used here. To ensure
  existing functionality compatibility, I have made the use of
  rbenv-root conditional on the nonexistence of $RBENV_ROOT.