Put the following files in a folder called `bin` (so they'll be `bin/post_compile` and `bin/install_nodejs`)

install_nodejs

#!/usr/bin/env bash
set -eo pipefail

NODE_VERSION=$(curl --silent --get https://semver.io/node/stable)
NODE_BASENAME=node-v${NODE_VERSION}-linux-x64
NODE_ARCHIVE="http://nodejs.org/dist/v${NODE_VERSION}/${NODE_BASENAME}.tar.gz"

# make a temp directory
tempdir="$( mktemp -t node_XXXX )"
rm -rf $tempdir
mkdir -p $tempdir

pushd $tempdir
curl -s -L -o tmp-nodejs.tar.gz $NODE_ARCHIVE
tar -zxvf tmp-nodejs.tar.gz > /dev/null
rm tmp-nodejs.tar.gz
popd

mkdir -v -p $BUILD_DIR/.heroku/vendor
pushd $BUILD_DIR/.heroku/vendor
rm -rf node
mv $tempdir/$NODE_BASENAME node
popd

ln -sf ../../vendor/node/bin/node .heroku/python/bin/node
ln -sf ../../vendor/node/bin/node-waf .heroku/python/bin/node-waf
ln -sf ../../vendor/node/bin/npm .heroku/python/bin/npm

post_compile

#!/usr/bin/env bash
set -veo pipefail

# The post_compile hook is run by heroku-buildpack-python

echo "-----> Running post-compile hook"

# Work around Heroku bug whereby pylibmc isn't available during
# compile phase. See: https://github.com/heroku/heroku-buildpack-python/issues/57
export MEMCACHE_SERVERS='' MEMCACHIER_SERVERS=''

if [ -f bin/install_nodejs ]; then
    echo "-----> Running install_nodejs"
    chmod +x bin/install_nodejs
    bin/install_nodejs

    # PUT YOUR NPM INSTALL LINES HERE: (less and iced-coffee-script for example)
    #npm install -g less
    #npm install -g iced-coffee-script
fi

# PUT ANY POST-BUILD STUFF HERE (like database migrations)
#python app/manage.py syncdb --noinput
#python app/manage.py migrate --noinput

echo "-----> Post-compile done"

It looks like the MEMCACHE_SERVERS workaround isn't needed now, as of:
heroku/heroku-buildpack-python@b891049