https://www.bitrise.io/app/APP-SLUG/build/start.json endpoint is now deprecated in favor of
https://app.bitrise.io/app/APP-SLUG/build/start.json (the www subdomain changed to app).
For now both endpoints can be used. Starting from 12th April, 2018 the www version will redirect to the app subdomain, so if your client does not follow redirects, you might not be able to trigger builds using this endpoint. Eventually we will remove the www version.
More info on the Build Trigger API can be found here.