Correct Heroku doc regarding Varnish

Varnish is no longer part of Heroku's platform, be it Bamboo or Cedar.

Heroku removed Varnish from the Bamboo stack in Jan 2013:
https://devcenter.heroku.com/changelog-items/201
Merge pull request #270 from DynamoMTL/heroku-doc-fix

Correct Heroku doc regarding Varnish