Downloads wkhtmlto* binaries, verifies, then copies bin/
- wkhtmltopdf 0.12.3
$ heroku buildpacks:set https://github.com/issueapp/heroku-buildpack-wkhtmltopdf
$ heroku config:set WKHTMLTOPDF_URL=http://download.gna.org/wkhtmltopdf/0.12/0.12.3/{wkhtmltox-0.12.3_linux-generic-amd64.tar.xz,SHA1SUMS}
$ git commit --allow-empty
$ git push heroku
$ heroku run bin/wkhtmltopdf -V
# profit!