If a gem is depending on hiredis during gem install (such as recommendify), git push heroku master fails since Heroku slug compiler doesn't have hiredis by default. In this case, you need to add hiredis first and setup proper PATHs before you run bundle install. This buildpack is in charge of doing it.
heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
cat << EOF > .buildpacks
https://github.com/keiko713/heroku-buildpack-hiredis.git
https://github.com/heroku/heroku-buildpack-ruby.git
EOF