update.sh: use sort instead of git tag --sort.

Older versions of Git don't have this flag and we don't want to block
updates for them when there's a (relatively) simple workaround.

Library/Homebrew/cmd/update.sh

@@ -217,7 +217,8 @@ merge_or_rebase() {
 
   if [[ "$DIR" = "$HOMEBREW_REPOSITORY" && -n "$HOMEBREW_UPDATE_TO_TAG" ]]
   then
-    UPSTREAM_TAG="$(git tag --list --sort=-version:refname |
+    UPSTREAM_TAG="$(git tag --list |
+                    sort --field-separator=. --key=1,1nr -k 2,2nr -k 3,3nr |
                     grep --max-count=1 '^[0-9]*\.[0-9]*\.[0-9]*$')"
   else
     UPSTREAM_TAG=""