As per the Heroku support staff, this is the magic thing:
heroku config:add LANG=en_US.UTF-8
Although heroku console will keep reporting strings encoding as ASCII-8BIT, your actuall app will be running with the correct encoding, based on the LANG config var. You can double check that by doing this:
$ heroku run bash Running bash attached to terminal... up, run.2 u20415@022e95bf-3ab6-4291-97b1-741f95e7fbda:/app$ irb irb(main):001:0> "a".encoding => #<Encoding:UTF-8>
