Update manage-packages.el: Fix "Failed to download `gnu' archive."

Without this patch the following will happen:

/tmp/install-melpa.sh gnuplot    
Loading /etc/emacs/site-start.d/00debian.el (source)...
Contacting host: melpa.org:443
Failed to download `melpa' archive.
Contacting host: elpa.gnu.org:443
Package refresh done
Failed to download `gnu' archive.
Parsing tar file...
Malformed Tar header