update default installation to Node 12 for new active version
update default installation to Node 12 for new active version

Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack