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
