bug: non-fatal `curl: unknown --write-out variable: 'url'` in deployment

[mcdurdin]

For example:

curl: unknown --write-out variable: 'url'
curl: Finished attempt to download [bootstrap] Bootstrap required
[bootstrap] Bootstrap starting
[bootstrap]   Downloading bootstrap.inc.sh

This looks like bootstrap is using --write-out with 'url' variable introduced in a curl version later than the one we have in our deployed containers. Either change the format string or update curl?