Response enhancement for in the case of the working directory under the UNC path.

finding_local_version_file is extremely slow, when working directory is under the UNC path.
Because //host/.rbenv-version and //.rbenv-version do not exist, but testing them is so slow.
It's the reason to make a serious delay of the response, when the Ruby runs with a current working directory under the UNC path under Cygwin environment.

A response of before applying this patch.
//somehost/somedir $ time ruby -e "exit"

real    0m13.922s
user    0m0.168s
sys     0m0.287s

A response of after applying this patch.
//somehost/somedir $ time ruby -e "exit"

real    0m0.721s
user    0m0.153s
sys     0m0.319s

libexec/rbenv-version-file

@@ -5,7 +5,7 @@ set -e
 
 find_local_version_file() {
   local root="$1"
-  while [ -n "$root" ]; do
+  while [[ -n "$root" && ! "$root" =~ ^//[^/]*$ ]]; do
     if [ -e "${root}/.ruby-version" ]; then
       echo "${root}/.ruby-version"
       exit