Not tested, but looking at the docstring of bibtex-entry-format, it says:
bibtex-entry-format is a variable defined in bibtex.el. Its value is
(opts-or-alts required-fields numerical-fields)
Type of formatting performed by bibtex-clean-entry.
It may be t, nil, or a list of symbols out of the following:
sort-fields Sort fields to ...
The following snippet works for me. I use use-package, so it is slightly different, but I hope it is easy enough to edit. You can evaluate this with emacs -q and then calling M-x eval-buffer on the snippet.
(setq package-user-dir (expand-file-name "~/.emacs.d/elpa/")