Fix discovering `.ruby-version` files in root directory

It's not that this is a preferred way to set a global version (one
should use `rbenv global <version>` instead), but this fixes the
function purely for correctness: all parent directories should be
scanned, even the root directory.

Fixes #745

libexec/rbenv-version-file

@@ -5,7 +5,7 @@ set -e
 
 find_local_version_file() {
   local root="$1"
-  while [ -n "$root" ]; do
+  while true; do
     if [ -e "${root}/.ruby-version" ]; then
       echo "${root}/.ruby-version"
       exit
@@ -13,6 +13,7 @@ find_local_version_file() {
       echo "${root}/.rbenv-version"
       exit
     fi
+    [ -n "$root" ] || break
     root="${root%/*}"
   done
 }