Do not initialize RBENV_ROOT to early

I'm not sure, why it broke for me just right now after an upgrade (which I'm doing frequently). 

Since $RBENV_ROOT gets initialized with the rbenv installation dir in $HOME on line 14  it cannot be empty for the check on line 21. So its fixed on $HOME/.rbenv.
Here is a patch removing the conditional and initialization on lines 13 to 15, so that the for loop can do the detection as I guess was intended.

Kind regards, Thomas