I'd like to add to the accepted answer with a bit more explanation since I had this exact same issue and I believe most folks will need to unset an old buildpack as follows:
First unset your old buildpack and point to the buildpack-multi:
heroku config:unset BUILDPACK_URL
heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
Buildpack multi requires you to add your own .buildpacks file. This configuration is what I ended up using successfully:
cat .buildpacks
https://github.com/heroku/heroku-buildpack-nodejs.git
https://github.com/heroku/heroku-buildpack-ruby.git
Here's my Gemfile to pickup only Compass:
cat Gemfile
source 'https://rubygems.org'
gem 'compass'
Now you need to do:
bundle install
Which will add a Gemfile.lock
Commit everything to git and push back to heroku. This will kick in your new multi buildpack configuration and hopefully get you back up and running. You should see both the nodejs and ruby buildpacks download serially per above configuration.
Disclaimer: This is likely time-sensitive material as heroku might very well change something over the upcoming months.