Remove Heroku from README

Heroku no longer recommend using Asset Sync for managing assets.  This is because:

- It extends out the build process time, frequently causing timeouts
- Requires use of labs features
- Doesn't work with the new releases API

As such the asset sync article on Devcenter has been replaced with ["Using Amazon CloudFront CDN with Rails"](https://devcenter.heroku.com/articles/using-amazon-cloudfront-cdn-with-rails)

Ideally, we don't want to encourage newcomers to Heroku to use this as it invariably ends up with problems.  Therefore, could we remove the Heroku-ness from the README?  Whilst asset_sync is useful for others, it's no ideal for Heroku customers.