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.
Merge pull request #2 from halostatue/fix-update-error-without-RBENV_ROOT

rbenv-update fails without RBENV_ROOT
Call git fetch origin before pull to avoid "Your branch is ahead of 'origin/master' by 22 commits." style messages
Merge pull request #3 from ianheggie/fix_ahead_of_origin_master_warnings

Fixes "Your branch is ahead of ''origin/master' by 22 commits." style messages
Merge branch 'master' of github.com:rkh/rbenv-update