Use the option package-archive-priorities which was made for this
very purpose. To prefer MELPA Stable over MELPA.
The higher the number, the higher the priority of a package archive.
For any package that exists in multiple archives Emacs' package manager
picks the package from the archive with the highest priority,
and ignores packages in other archives, even if the version number
is higher.
If the package exists in two archives of the same priority,
the higher version number wins, and ultimately
I think the order in package-archives.
