Add Heroku nodejs buildpack

**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.

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
     },
@@ -83,14 +71,12 @@
     }
   },
   "addons": [
-    "logentries",
-    "newrelic",
     "memcachier",
-    "heroku-postgresql",
     "sendgrid"
   ],
   "buildpacks": [
     {
+      "url": "heroku/nodejs",
       "url": "heroku/ruby"
     }
   ]