With DNSimple you can request an SSL certificate that you can install on Heroku to enable HTTPS on your Heroku application. DNSimple also provides the ALIAS feature that is necessary if you want to point your root domain to Heroku, as explained in the official Heroku documentation.
This article provides step-by-step instructions to obtain a new SSL certificate via DNSimple, install it on Heroku, and configure Heroku to use the new SSL certificate with the appropriate domain.
On that page, click the Heroku section and follow the directions presented to you.