Add 'nodejs' buildback (Ref: rails/webpacker#739)

Procfile

@@ -1,2 +1,2 @@
-rails: rails server -p 3333
-webpack: ./bin/webpack-dev-server
+release: bin/rails db:migrate; NODE_ENV=production bin/rails webpacker:compile
+web: bin/rails server -p $PORT -e $RAILS_ENV

Procfile.dev

@@ -0,0 +1,2 @@
+rails: rails server -p 3333
+webpack: ./bin/webpack-dev-server

app.json

@@ -2,8 +2,7 @@
   "name": "elmsanity",
   "description": "",
   "scripts": {
-    "postdeploy": "cp config/database.yml.postgresql config/database.yml; bundle exec rails db:create; bundle exec rails db:migrate; NODE_ENV=production bundle exec rails webpacker:compile"
-  },
+},
   "env": {
     "SECRET_KEY_BASE": {
       "generator": "secret"
@@ -19,6 +18,7 @@
 
   ],
   "buildpacks": [
-
+    { "url": "heroku/nodejs" },
+    { "url": "heroku/ruby" }
   ]
 }