that’s what I get executing (require 'org-ref):

Debugger entered--Lisp error: (error "Required feature ‘let-alist’ was not provided")
  require(let-alist)
  byte-code("\300\301!\210\300\302!\210\300\303!\210\300\304!\210\300\305!\210\300\306!\210\300\307!\210\300\310!\210\300\311!\207" [require bibtex browse-url hl-line ido json url-queue dash let-alist seq] 2)
  require(biblio-core)
  byte-code("\300\301!\210\300\302!\210\300\303!\210\300\304!\210\300\305!\210\300\306!\210\300\307!\210\300\310!\210\311\312!\207" [require biblio-core biblio-doi biblio-arxiv biblio-crossref biblio-dblp biblio-hal biblio-dissemin biblio-download provide biblio] 2)
  require(biblio)
  eval-buffer(#<buffer  *load*-844603> nil "/Users/johannes/Library/Preferences/Aquamacs Emacs/Packages/elpa/helm-bibtex-20170321.1306/bibtex-completion.el" nil t)  ; Reading at buffer position 1340
  load-with-code-conversion("/Users/johannes/Library/Preferences/Aquamacs Emacs/Packages/elpa/helm-bibtex-20170321.1306/bibtex-completion.el" "/Users/johannes/Library/Preferences/Aquamacs Emacs/Packages/elpa/helm-bibtex-20170321.1306/bibtex-completion.el" nil t)
  require(bibtex-completion)
  byte-code("\306\307!\210\306\310!\210\306\311!\210\306\312!\210\313 \314\211  \314  :\203f