Need for ~/.rbenvrc

Variables like `$RBENV_ROOT` have to be set somewhere if you can't live with defaults. As long as you use Bash as your login shell, you can set them in `.bash_profile`. But in other cases, you have to stick to defaults. Since it's non-interactive shell, `.bashrc` works neither.

Quite recently, I had to hand my laptop to someone else for a sec. If I only could set  $RBENV_ROOT to some shared location, I could create a new user who would use the same rubies as I do. Also, Homebrew suggests setting RBENV_ROOT to `/usr/local/var/rbenv`.