Downloads wkhtmlto* binaries, verifies, then copies bin/
- wkhtmltopdf 0.12.4
$ heroku buildpacks:set https://github.com/issueapp/heroku-buildpack-wkhtmltopdf
$ heroku config:set WKHTMLTOPDF_URL=https://downloads.wkhtmltopdf.org/0.12/0.12.4/{wkhtmltox-0.12.4_linux-generic-amd64.tar.xz,SHA1SUMS}
$ git commit --allow-empty
$ git push heroku
$ heroku run bin/wkhtmltopdf -V
# profit!