Render-readmes script should fall back to checking crate.readme in the database for the most recent version

[carols10cents]

There are some crates that have a readme specified in Cargo.toml, but exclude the README.md file from the archive (see #968). So render-readmes can't find the README.md in the .crate files from s3.

HOWEVER, we happen to have in the database the most recently published (which should probably be latest semvertically, see #953, #894) README.md that's used for searching. We could at least check that before giving up.