I got it to work! The solution was setting GEM_HOME in the node buildpack to the directory the ruby buildpack installed the gems into, then adding that dir's bin and the ruby executable location to the PATH:
export GEM_HOME=$build_dir/vendor/bundle/ruby/2.0.0/
PATH=$GEM_HOME/bin:$build_dir/vendor/bundle/bin:$build_dir/vendor/ruby-2.0.0/bin:$PATH