Add Heroku nodejs buildpack to app.json

**Why**: After upgrading to the latest webpacker, assets failed to
compile on Heroku. This issue says you need to add the nodejs buildpack:
rails/webpacker#739

Make sure the nodejs buildpack comes before the ruby one.
For existing apps, this can be done like so:
heroku buildpacks:add heroku/nodejs -i 1 -a your_app_name

I also removed unnecessary items from app.json.

app.json

@@ -24,9 +24,6 @@
     "GOOGLE_ANALYTICS_ID": {
       "required": true
     },
-    "HEROKU_POSTGRESQL_RED_URL": {
-      "required": true
-    },
     "LANG": {
       "required": true
     },
@@ -42,15 +39,6 @@
     "MEMCACHIER_USERNAME": {
       "required": true
     },
-    "NEW_RELIC_APP_NAME": {
-      "required": true
-    },
-    "NEW_RELIC_LICENSE_KEY": {
-      "required": true
-    },
-    "NEW_RELIC_LOG": {
-      "required": true
-    },
     "OHANA_API_ENDPOINT": {
       "required": true
     },
@@ -83,13 +71,13 @@
     }
   },
   "addons": [
-    "logentries",
-    "newrelic",
     "memcachier",
-    "heroku-postgresql",
     "sendgrid"
   ],
   "buildpacks": [
+    {
+      "url": "heroku/nodejs"
+    },
     {
       "url": "heroku/ruby"
     }

script/setup_heroku

@@ -59,6 +59,9 @@ heroku addons:create sendgrid:starter --app $APP_NAME
 echo "Installing Memcachier"
 heroku addons:create memcachier --app $APP_NAME
 
+echo "Adding NodeJS buildback for compatibility with webpacker"
+heroku buildpacks:add heroku/nodejs -i 1 --app $APP_NAME
+
 echo "All done setting up env vars and add-ons."
 echo "Pushing code to Heroku now. This will take a few minutes..."
 git push heroku master