Create a connected app in your Spring '15 org with callback URL
Wait for 2-10 minutes for the connected app data to replicate around Salesforce.
Deploy the demo code to Heroku:
In your Spring '15 org, go to Setup | Security Controls | CORS and add
https://your-app.herokuapp.com as a Whitelisted Origin.
https://your-app.herokuapp.com/, authorize the app to access your Spring '15 org, and try uploading a file!
If you see an error such as
check that you have copied the Consumer Key from the connected app to the
CLIENT_ID environment variable correctly. If all looks good, go make a cup of tea - Salesforce probably needs a couple more minutes to propagate the connected app data.