Flask's send_file uses mimetypes.guess_type() which needs the file extension
at the end of the path. Previously files were named acme.sh-3.1.3 and
htm.js-3.1.1 — now acme-3.1.3.sh and htm-3.1.1.js.
Also: update-vendor.sh now owns ACME_HOME installation, removing duplicate
logic from install.sh.
