switch order of rbenv formula caveats

The caveats said to add

    if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi

to your profile. Then to add

    export RBENV_ROOT=#{var}/rbenv

to your profile.

Those instructions should be in the reverse order.
The rbenv root path needs to be set before enabling rbenv shims.

    export RBENV_ROOT=#{var}/rbenv
    if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi

Otherwise `rbenv init` will place `~/.rbenv/shims` in the `$PATH`
rather than `#{var}/rbenv/shims`.