pyenv-version-file-read: use POSIX character sets (#1554)

Fixes #623

libexec/pyenv-version-file-read

@@ -9,7 +9,7 @@ if [ -e "$VERSION_FILE" ]; then
   # Read the first non-whitespace word from the specified version file.
   # Be careful not to load it whole in case there's something crazy in it.
   IFS="${IFS}"$'\r'
-  words=($(cut -b 1-1024 "$VERSION_FILE" | sed 's/^\s*\(\S\+\).*/\1/'))
+  words=($(cut -b 1-1024 "$VERSION_FILE" | sed 's/[[:space:]]*\([^[:space:]^[:space:]]*\).*/\1/'))
   versions=("${words[@]}")
 
   if [ -n "$versions" ]; then